U.S. patent application number 16/355454 was filed with the patent office on 2019-07-11 for nanostructured battery active materials and methods of producing same.
The applicant listed for this patent is OneD Material LLC. Invention is credited to Wanqing Cao, Virginia Robbins, Yimin Zhu.
Application Number | 20190214641 16/355454 |
Document ID | / |
Family ID | 47601746 |
Filed Date | 2019-07-11 |
United States Patent
Application |
20190214641 |
Kind Code |
A1 |
Cao; Wanqing ; et
al. |
July 11, 2019 |
Nanostructured Battery Active Materials and Methods of Producing
Same
Abstract
Methods for producing nanostructures from copper-based catalysts
on porous substrates, particularly silicon nanowires on
carbon-based substrates for use as battery active materials, are
provided. Related compositions are also described. In addition,
novel methods for production of copper-based catalyst particles are
provided. Methods for producing nanostructures from catalyst
particles that comprise a gold shell and a core that does not
include gold are also provided.
Inventors: |
Cao; Wanqing; (Fremont,
CA) ; Robbins; Virginia; (Los Gatos, CA) ;
Zhu; Yimin; (Union City, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
OneD Material LLC |
Palo Alto |
CA |
US |
|
|
Family ID: |
47601746 |
Appl. No.: |
16/355454 |
Filed: |
March 15, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14234565 |
Jul 24, 2014 |
10243207 |
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PCT/US2012/047979 |
Jul 24, 2012 |
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16355454 |
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61511826 |
Jul 26, 2011 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/0428 20130101;
H01M 4/622 20130101; H01M 4/366 20130101; C01P 2004/16 20130101;
C30B 25/005 20130101; C30B 11/12 20130101; B01J 23/72 20130101;
C30B 29/06 20130101; C30B 29/60 20130101; C01P 2004/64 20130101;
H01M 4/386 20130101; B01J 37/0211 20130101; C01B 33/021 20130101;
H01M 4/134 20130101; C01B 33/02 20130101; H01M 4/0495 20130101;
B01J 21/18 20130101; H01M 4/36 20130101; C01B 33/029 20130101 |
International
Class: |
H01M 4/36 20060101
H01M004/36; C30B 29/60 20060101 C30B029/60; C30B 11/12 20060101
C30B011/12; C30B 25/00 20060101 C30B025/00; C01B 33/029 20060101
C01B033/029; B01J 37/02 20060101 B01J037/02; B01J 23/72 20060101
B01J023/72; B01J 21/18 20060101 B01J021/18; H01M 4/62 20060101
H01M004/62; C30B 29/06 20060101 C30B029/06; H01M 4/04 20060101
H01M004/04; H01M 4/38 20060101 H01M004/38; C01B 33/021 20060101
C01B033/021; C01B 33/02 20060101 C01B033/02; H01M 4/134 20060101
H01M004/134 |
Claims
1-82. (canceled)
83. A method for depositing Cu.sub.2O nanoparticles on the surface
of a carbon-based porous substrate, the method comprising:
providing a carbon-based porous substrate comprising a population
of particles comprising at least one of natural graphite particles,
synthetic graphite particles, graphene particles, carbon fibers,
carbon nanostructures, carbon nanotubes, or carbon black; first
mixing deionized water with a copper source and a chelating agent
to form a first aqueous solution comprising copper ions, wherein
the copper source comprises at least one of copper sulfate, copper
nitrate, copper chloride, or copper acetate and wherein the
chelating agent comprises at least one of Rochelle salt (i.e.
potassium sodium tartrate), EDTA, or polyols; second mixing
deionized water with a reducing agent to form a second aqueous
solution, wherein the reducing agent comprises at least one of
sodium ascorbate, or ascorbic acid; mixing the first aqueous
solution with the second aqueous solution to form Cu.sub.2O
colloidal nanoparticles in an alkaline plating solution, the
Cu.sub.2O colloidal nanoparticles formed via chemical reduction of
the copper source, and wherein the mixing comprises setting the pH
of the alkaline plating solution and setting the concentration of
the copper ions in the alkaline plating solution to control the
stability, the size distribution and the average size of the formed
Cu.sub.2O colloidal nanoparticles; immersing the carbon-based
porous substrate into the alkaline plating solution comprising the
formed Cu.sub.2O colloidal nanoparticles; depositing the formed
Cu.sub.2O colloidal nanoparticles onto the surface of the
carbon-based porous substrate until the alkaline plating solution
is substantially completely depleted of the copper ions; and
removing the carbon-based porous substrate with the Cu.sub.2O
nanoparticles deposited thereon from the plating solution and
drying the carbon-based porous substrate in an oven.
84. The method of claim 83 further comprising, prior to immersing,
setting the copper ions concentration to at most 10 millimolar.
85. The method of claim 84 further comprising, prior to immersing,
setting the pH of the alkaline plating solution between 8 and 11
and setting the copper ions concentration up to 5 millimolar.
86. The method of claim 83, wherein the particles in the population
of particles in the carbon-based porous substrate have an average
diameter between 0.5 .mu.m and 50 .mu.m.
87. The method of claim 83, wherein the particles in the population
of particles in the carbon-based porous substrate have an average
diameter between 2 .mu.m and 10 .mu.m.
88. The method of claim 83, wherein depositing further comprises
depositing the formed Cu.sub.2O colloidal nanoparticles onto the
surface of the carbon-based porous substrate until the
concentration of copper ions in the alkaline plating solution is
less than 1 ppm.
89. The method of claim 83, wherein the formed Cu.sub.2O
nanoparticles in the alkaline plating solution have an average size
between 5 nm and 100 nm as measured by a light scattering
measurement or by electron microscopy.
90. The method of claim 83, wherein the formed Cu.sub.2O
nanoparticles in the alkaline plating solution have an average size
between 20 nm and 50 nm as measured by a light scattering
measurement or by electron microscopy.
91. The method of claim 83, wherein the formed Cu.sub.2O
nanoparticles in the alkaline plating solution have an average size
between 20 nm and 40 nm as measured by a light scattering
measurement or by electron microscopy.
92. The method of claim 83, wherein removing further comprises
filtering the plating solution to recover the carbon-based porous
substrate with the Cu.sub.2O nanoparticles deposited thereon and
drying the carbon-based porous substrate in an oven.
93. A method for producing nanostructures on a carbon-based porous
substrate, the method comprising: providing a carbon-based porous
substrate comprising a population of particles comprising at least
one of natural graphite particles, synthetic graphite particles,
graphene particles, carbon fibers, carbon nanostructures, carbon
nanotubes, or carbon black; first mixing deionized water with a
copper source and a chelating agent to form a first aqueous
solution comprising copper ions, wherein the copper source
comprises at least one of copper sulfate, copper nitrate, copper
chloride, or copper acetate and wherein the chelating agent
comprises at least one of Rochelle salt (i.e. potassium sodium
tartrate), EDTA, or polyols; second mixing deionized water with a
reducing agent to form a second aqueous solution, wherein the
reducing agent comprises at least one of sodium ascorbate, or
ascorbic acid; mixing the first aqueous solution with the second
aqueous solution to form Cu.sub.2O colloidal nanoparticles in an
alkaline plating solution, the Cu.sub.2O colloidal nanoparticles
formed via chemical reduction of the copper source, and wherein the
mixing comprises setting the pH of the alkaline plating solution
and setting the concentration of the copper ions in the alkaline
plating solution to control the stability, the size distribution
and the average size of the formed Cu.sub.2O colloidal
nanoparticles; immersing the carbon-based porous substrate into the
alkaline plating solution comprising the formed Cu2O colloidal
nanoparticles; depositing the formed Cu.sub.2O colloidal
nanoparticles onto the surface of the carbon-based porous substrate
until the alkaline plating solution is substantially completed
depleted of the copper ions; removing the carbon-based porous
substrate with the Cu.sub.2O nanoparticles deposited thereon from
the plating solution and drying the carbon-based porous substrate
in an oven; loading the carbon-based porous substrate into a
reaction vessel wherein the population of particles with the
Cu.sub.2O nanoparticles deposited thereon form a packed bed in the
reaction vessel; and growing nanostructures on the carbon-based
porous substrate in the reaction vessel from the Cu.sub.2O
nanoparticles via a Vapor-Solid-Solid (VSS) synthesis technique,
wherein the growing comprises mixing the packed bed while flowing
one or more reactant gases in the reaction vessel during the
nanostructure growing process; and wherein the nanostructures
comprise silicon, germanium or a combination thereof.
94. The method of claim 93, wherein the nanostructures comprise at
least one of nanowires or nanoparticles.
95. The method of claim 93, wherein the one or more reactant gas
comprise silane (SiH.sub.4).
96. The method of claim 93, further comprising, after growing,
applying a carbon coating or an oxide coating to the
nanostructures.
97. The method of claim 93, further comprising, after growing,
incorporating the carbon-based porous substrate with the
nanostructure grown thereon into a battery slurry.
98. The method of claim 93, further comprising, after growing,
incorporating the carbon-based porous substrate with the
nanostructure grown thereon into a battery anode.
99. The method of claim 93, further comprising, after growing,
incorporating the carbon-based porous substrate with the
nanostructure grown thereon into a battery.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Application No. 61/511,826, filed Jul. 26, 2011, which is
incorporated herein by reference in its entirety.
FIELD OF THE INVENTION
[0002] The invention pertains to the field of nanotechnology. More
particularly, the invention relates to methods for producing
nanostructures from copper-based catalyst materials, particularly
silicon nanostructures on carbon-based substrates for use as
battery active materials. The invention also relates to
compositions including silicon nanowires on porous substrates,
particularly carbon-based substrates that can serve as battery
active materials.
BACKGROUND OF THE INVENTION
[0003] Silicon nanowires are desirable materials for many
applications in the semiconductor industry, as well as in
production of medical devices and high capacity lithium-ion
batteries. Gold nanoparticles have been extensively used to
catalyze growth of silicon nanowires. However, the cost of gold
becomes significant or even prohibitive for large scale synthesis
of silicon nanowires, and gold is not compatible with all desired
applications for the nanowires.
[0004] Methods for silicon nanostructure growth that reduce or even
eliminate the need for a gold catalyst are thus desirable. Among
other aspects, the present invention provides such methods. A
complete understanding of the invention will be obtained upon
review of the following.
SUMMARY OF THE INVENTION
[0005] Methods for producing nanostructures from copper-based
catalysts on porous substrates, particularly silicon nanowires on
carbon-based substrates for use as battery active materials, are
provided. Compositions including such nanostructures are described.
Novel methods for production of copper-based catalyst particles are
also provided.
[0006] One general class of embodiments provides methods for
producing nanostructures. In the methods, a porous substrate having
catalyst particles disposed thereon is provided, and the
nanostructures are grown from the catalyst particles. The catalyst
particles comprise copper, a copper compound, and/or a copper
alloy.
[0007] The substrate can comprise, e.g., a carbon-based substrate,
a population of particles, a population of graphite particles, a
plurality of silica particles, a plurality of carbon sheets, carbon
powder, natural and/or artificial graphite, graphene, graphene
powder, carbon fibers, carbon nanostructures, carbon nanotubes,
carbon black, a mesh, or a fabric. In one class of embodiments, the
substrate comprises a population of graphite particles and the
nanostructures are silicon nanowires.
[0008] The catalyst particles can be of essentially any desired
size but are typically nanoparticles. For example, the catalyst
particles optionally have an average diameter between about 5 nm
and about 100 nm, e.g., between about 20 nm and about 50 nm, e.g.,
between about 20 nm and about 40 nm.
[0009] As noted above, the catalyst particles can comprise copper,
a copper compound, and/or a copper alloy. For example, the catalyst
particles can comprise copper oxide. In one class of embodiments,
the catalyst particles comprise copper (I) oxide (Cu.sub.2O),
copper (II) oxide (CuO), or a combination thereof. In one class of
embodiments, the catalyst particles comprise elemental (i.e.,
pure-phase) copper (Cu), copper (I) oxide (Cu.sub.2O), copper (II)
oxide (CuO), or a combination thereof In another class of
embodiments, the catalyst particles comprise copper acetate, copper
nitrate, or a copper complex comprising a chelating agent (e.g.,
copper tartrate or copper EDTA).
[0010] Catalyst particles can be produced and disposed on the
substrate by essentially any convenient techniques, including, but
not limited to, colloidal synthesis followed by deposition,
adsorption of copper ions or complexes, and electroless deposition.
Thus, in one class of embodiments, providing a porous substrate
having catalyst particles disposed thereon comprises synthesizing
colloidal nanoparticles comprising copper and/or a copper compound
and then depositing the nanoparticles on the substrate. The
nanoparticles optionally comprise elemental copper (Cu), copper (I)
oxide (Cu.sub.2O), copper (II) oxide (CuO), or a combination
thereof, and the substrate optionally comprises a population of
graphite particles. In another class of embodiments, providing a
porous substrate having catalyst particles disposed thereon
comprises synthesizing discrete particles on the substrate through
electroless deposition of copper onto the substrate, by immersing
the substrate in an electroless plating solution comprising copper
ions (e.g., at most 10 millimolar copper ions) and a reducing agent
(e.g., formaldehyde). The plating solution is typically alkaline.
The substrate optionally comprises a population of graphite
particles. In another class of embodiments, providing a porous
substrate having catalyst particles disposed thereon comprises
immersing the porous substrate in a solution comprising copper ions
and/or a copper complex, whereby the copper ions and/or the copper
complex are adsorbed on the surface of the substrate, thereby
forming discrete nanoparticles on the surface of the substrate. The
solution is typically an aqueous alkaline solution. The substrate
optionally comprises a population of graphite particles.
[0011] The methods can be used to synthesize essentially any
desired type of nanostructures, including, but not limited to,
nanowires. The nanowires can be of essentially any desired size.
For example, the nanowires can have an average diameter less than
about 150 nm, e.g., between about 10 nm and about 100 nm, e.g.,
between about 30 nm and about 50 nm.
[0012] The nanostructures can be produced from any suitable
material, including, but not limited to, silicon. In embodiments in
which the nanostructures comprise silicon, the nanostructures can
comprise, e.g., monocrystalline silicon, polycrystalline silicon,
amorphous silicon, or a combination thereof. Thus, in one class of
embodiments, the nanostructures comprise a monocrystalline core and
a shell layer, wherein the shell layer comprises amorphous silicon,
polycrystalline silicon, or a combination thereof. In one aspect,
the nanostructures are silicon nanowires.
[0013] The nanostructures can be grown using essentially any
convenient technique. For example, silicon nanowires can be grown
via a vapor-liquid-solid (VLS) or vapor-solid-solid (VSS)
technique.
[0014] The methods can be employed for production of nanostructures
for use in any of a variety of different applications. For example,
the nanostructures and the substrate on which they were grown can
be incorporated into a battery slurry, battery anode, and/or
battery, e.g., a lithium ion battery.
[0015] In one class of embodiments, the substrate comprises a
population of graphite particles and the nanostructures comprise
silicon nanowires, and silicon comprises between 2% and 20% of the
total weight of the nanostructures and the graphite particles after
nanostructure growth is completed.
[0016] Another general class of embodiments provides methods for
producing silicon nanowires. In the methods, colloidal
nanoparticles comprising copper and/or a copper compound are
synthesized and deposited on a substrate, and the nanowires are
grown from the nanoparticles.
[0017] The copper compound is optionally copper oxide. In one class
of embodiments, the nanoparticles comprise elemental copper (Cu),
copper (I) oxide (Cu.sub.2O), copper (II) oxide (CuO), or a
combination thereof. The size of the nanoparticles can vary, for
example, depending on the diameter desired for the resulting
nanowires. For example, the nanoparticles optionally have an
average diameter between about 5 nm and about 100 nm, e.g., between
about 10 nm and about 100 nm, between about 20 nm and about 50 nm,
or between about 20 nm and about 40 nm.
[0018] Essentially all of the features noted for the embodiments
above apply to these embodiments as well, as relevant; for example,
with respect to type and composition of substrate (e.g., a
population of graphite particles), nanostructure growth technique
(e.g., VLS or VSS), type, composition, and size of the resulting
nanostructures, ratio of nanostructures to substrate (e.g., silicon
to graphite) by weight, incorporation into a battery slurry,
battery anode, or battery, and/or the like.
[0019] Another general class of embodiments provides methods for
producing nanoparticles by electroless deposition. In the methods,
a substrate is provided. Also provided is an electroless plating
solution that comprises at most 10 millimolar copper ions (e.g.,
Cu.sup.2+ and/or Cu.sup.+). The substrate is immersed in the
plating solution, whereby the copper ions from the plating solution
form discrete nanoparticles comprising copper and/or a copper
compound on the substrate, until the plating solution is
substantially completely depleted of copper ions.
[0020] Suitable substrates include planar substrates, silicon
wafers, foils, and nonporous substrates, in addition to porous
substrates such as those described above, e.g., a population of
particles, e.g., a population of graphite particles.
[0021] The substrate is typically activated prior to its immersion
in the electroless plating solution. The substrate is optionally
activated by soaking it in a solution of a metal salt, e.g.,
PdCl.sub.2 or AgNO.sub.3. Graphite substrates, however,
particularly graphite particles which have a high surface area, are
conveniently activated simply by heating them prior to immersion in
the plating solution. Thus, in one class of embodiments the
substrate comprises a population of graphite particles, which are
activated by heating to 20.degree. C. or more (preferably
40.degree. C. or more) prior to immersion in the plating
solution.
[0022] In embodiments in which the substrate comprises a population
of particles, the methods can include filtering the plating
solution to recover the substrate particles from the plating
solution after the plating solution is substantially completely
depleted of copper ions.
[0023] The plating solution can include a copper salt, e.g., a
copper (II) salt, as the copper source. The plating solution can
include, e.g., one or more of Rochelle salt, EDTA, and
N,N,N',N'-tetrakis (2-hydroxypropyl) ethylene-diamine) as a
chelating agent. The plating solution can include, e.g.,
formaldehyde or sodium hypophosphite as the reducing agents. In one
exemplary class of embodiments, the plating solution comprises a
copper (II) salt, Rochelle salt, and formaldehyde and has an
alkaline pH.
[0024] As noted, the resulting nanoparticles can include copper or
a copper compound (for example, copper oxide). In one class of
embodiments, the nanoparticles comprise elemental copper (Cu),
copper (I) oxide (Cu.sub.2O), copper (II) oxide (CuO), or a
combination thereof. The resulting nanoparticles optionally have an
average diameter between about 5 nm and about 100 nm, e.g., between
about 10 nm and about 100 nm, between about 20 nm and about 50 nm,
or between about 20 nm and about 40 nm.
[0025] The resulting nanoparticles are optionally employed as
catalyst particles for subsequent synthesis of other
nanostructures, e.g., nanowires. Thus, the methods can include,
after the plating solution is substantially completely depleted of
copper ions, removing the substrate from the plating solution and
then growing nanostructures (e.g., nanowires, e.g., silicon
nanowires) from the nanoparticles on the substrate.
[0026] Essentially all of the features noted for the embodiments
above apply to these embodiments as well, as relevant; for example,
with respect to nanostructure growth technique (e.g., VLS or VSS),
type, composition, and size of the resulting nanostructures, ratio
of nanostructures to substrate (e.g., silicon to graphite) by
weight, incorporation into a battery slurry, battery anode, or
battery, and/or the like.
[0027] The plating solution can be employed as a single use bath or
as a reusable bath. Thus, in one class of embodiments, after the
plating solution is substantially completely depleted of copper
ions, the substrate is removed from the plating solution, then
copper ions are added to the plating solution (e.g., by addition of
a copper (II) salt), and then a second substrate is immersed in the
plating solution. Typically, after addition of the copper ions, the
plating solution again comprises at most 10 millimolar copper ions.
The second substrate is typically but need not be of the same type
as the first substrate, e.g., a second population of particles,
e.g., graphite particles.
[0028] In embodiments in which the plating solution comprises
formaldehyde, after the plating solution is substantially
completely depleted of copper ions the formaldehyde can be treated
by addition of sodium sulfite to the plating solution prior to
disposing of the solution.
[0029] As noted, nanoparticles produced by electroless deposition
can be employed as catalyst particles in subsequent nanostructure
synthesis reactions. Accordingly, one general class of embodiments
provides methods for producing nanowires. In the methods, a
substrate is provided. An electroless plating solution comprising
copper ions is also provided, and the substrate is immersed in the
plating solution, whereby the copper ions from the plating solution
form discrete nanoparticles comprising copper and/or a copper
compound on the substrate. Nanowires are then grown from the
nanoparticles on the substrate.
[0030] Essentially all of the features noted for the embodiments
above apply to these embodiments as well, as relevant; for example,
with respect to type and composition of substrate (nonporous,
porous, particles, graphite particles, sheets, wafers, etc.),
activation of the substrate, size, shape, and composition of the
nanoparticles (e.g., elemental copper and/or copper oxide),
components of the plating solution (copper source and reducing,
chelating, and other reagents), filtration step to recover a
particulate substrate, reuse versus single use of the plating
solution, nanostructure growth technique (e.g., VLS or VSS), type,
composition, and size of the resulting nanostructures, ratio of
nanostructures to substrate (e.g., silicon to graphite) by weight,
incorporation into a battery slurry, battery anode, or battery,
and/or the like.
[0031] Nanoparticles produced by adsorption can be employed as
catalyst particles in subsequent nanostructure synthesis reactions.
Accordingly, another general class of embodiments provides methods
for producing silicon nanowires. In the methods, a substrate is
provided. A solution comprising copper ions and/or a copper complex
is also provided, and the substrate is immersed in the solution,
whereby the copper ions and/or the copper complex are adsorbed on
the surface of the substrate, thereby forming discrete
nanoparticles comprising a copper compound on the surface of the
substrate. The nanowires are then grown from the nanoparticles on
the substrate.
[0032] The solution optionally includes a copper (II) salt (e.g.,
copper sulfate, copper acetate, or copper nitrate) and/or a copper
complex comprising a chelating agent (e.g., copper (II) tartrate or
copper EDTA). The solution can be an aqueous solution, typically,
an alkaline solution.
[0033] The size of the nanoparticles can vary, for example,
depending on the diameter desired for the resulting nanowires. For
example, the nanoparticles optionally have an average diameter
between about 5 nm and about 100 nm.
[0034] Essentially all of the features noted for the embodiments
above apply to these embodiments as well, as relevant; for example,
with respect to type and composition of substrate (e.g., a
population of graphite particles), nanostructure growth technique
(e.g., VLS or VSS), composition and size of the resulting
nanowires, ratio of nanowires to substrate (e.g., silicon to
graphite) by weight, incorporation into a battery slurry, battery
anode, or battery, and/or the like.
[0035] Compositions produced by or useful in practicing any of the
methods herein are also a feature of the invention. Accordingly,
one general class of embodiments provides a composition that
includes a porous substrate and a population of silicon nanowires
attached thereto, wherein one end of a member nanowire is attached
to the substrate and the other end of the member nanowire comprises
copper, a copper compound, and/or a copper alloy.
[0036] Essentially all of the features noted for the embodiments
above apply to these embodiments as well, as relevant; for example,
with respect to type, composition, and size of nanostructures,
composition and configuration of the substrate, catalyst material,
incorporation into a battery slurry, battery anode, or battery,
and/or the like.
[0037] For example, the composition can include nanowires having an
average diameter between about 10 nm and about 100 nm, e.g.,
between about 30 nm and about 50 nm, e.g., between about 40 nm and
about 45 nm. The nanowires can comprise monocrystalline silicon,
polycrystalline silicon, amorphous silicon, or a combination
thereof For example, the nanowires optionally comprise a
monocrystalline core and a shell layer, wherein the shell layer
comprises amorphous silicon, polycrystalline silicon, or a
combination thereof.
[0038] As for the embodiments above, the porous substrate is
optionally a carbon-based substrate, a population of particles, a
plurality of silica particles, a plurality of carbon sheets, carbon
powder, natural and/or artificial graphite, a population of natural
and/or artificial graphite particles, graphene, graphene powder,
carbon fibers, carbon nanostructures, carbon nanotubes, carbon
black, a mesh, or a fabric.
[0039] The catalyst-derived material on the ends of the member
nanowires not attached to the substrate can comprise, e.g.,
elemental copper, copper oxide, copper silicide, or a combination
thereof.
[0040] The composition optionally includes a polymer binder, e.g.,
carboxymethyl cellulose. In one class of embodiments, the substrate
comprises a population of graphite particles, and silicon comprises
between 2% and 20% of the total weight of the nanostructures and
the graphite particles.
[0041] A battery slurry, battery anode, or battery comprising the
composition is also a feature of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0042] FIG. 1 Panels A and B show scanning electron micrographs of
colloidal Cu.sub.2O nanoparticles synthesized in an aqueous
medium.
[0043] FIG. 2 Panel A shows a scanning electron micrograph of
colloidal Cu.sub.2O nanoparticles deposited on a particulate
graphite substrate. Panel B shows a scanning electron micrograph of
silicon nanowires grown from the Cu.sub.2O nanoparticles on
graphite particles.
[0044] FIG. 3 Panel A shows a scanning electron micrograph of
copper nanoparticles deposited on a particulate graphite substrate
from an electroless plating solution. Panel B shows a scanning
electron micrograph of silicon nanowires grown from the copper
nanoparticles.
[0045] FIG. 4 Panel A schematically illustrates VLS growth of a
silicon nanowire from a gold catalyst particle. Panel B
schematically illustrates VLS growth of a silicon nanowire from a
non-gold core/gold shell catalyst particle. Panel C presents a
graph showing the percentage of the nanoparticle volume occupied by
the non-Au material (i.e., the volume of the core as a percentage
of the overall volume including both the core and the shell) for a
15 nm non-Au core coated with an Au shell of varying thickness.
[0046] FIG. 5 Panel A shows scanning electron micrographs of
nanoparticles deposited on a particulate graphite substrate by
electroless deposition (row I) and by adsorption (row II), at
increasing magnification from left to right. Panel B shows scanning
electron micrographs of silicon nanowires grown from the
nanoparticles produced by electroless deposition (row I) and
adsorption (row II), at increasing magnification from left to
right.
[0047] Schematic figures are not necessarily to scale.
DEFINITIONS
[0048] Unless defined otherwise, all technical and scientific terms
used herein have the same meaning as commonly understood by one of
ordinary skill in the art to which the invention pertains. The
following definitions supplement those in the art and are directed
to the current application and are not to be imputed to any related
or unrelated case, e.g., to any commonly owned patent or
application. Although any methods and materials similar or
equivalent to those described herein can be used in the practice
for testing of the present invention, the preferred materials and
methods are described herein. Accordingly, the terminology used
herein is for the purpose of describing particular embodiments
only, and is not intended to be limiting.
[0049] As used in this specification and the appended claims, the
singular forms "a," "an" and "the" include plural referents unless
the context clearly dictates otherwise. Thus, for example,
reference to "a nanostructure" includes a plurality of such
nanostructures, and the like.
[0050] The term "about" as used herein indicates the value of a
given quantity varies by +/-10% of the value, or optionally +/-5%
of the value, or in some embodiments, by +/-1% of the value so
described.
[0051] A "nanostructure" is a structure having at least one region
or characteristic dimension with a dimension of less than about 500
nm, e.g., less than about 200 nm, less than about 100 nm, less than
about 50 nm, or even less than about 20 nm. Typically, the region
or characteristic dimension will be along the smallest axis of the
structure. Examples of such structures include nanowires, nanorods,
nanotubes, nanofibers, branched nanostructures, nanotetrapods,
tripods, bipods, nanocrystals, nanodots, quantum dots,
nanoparticles, and the like. Nanostructures can be, e.g.,
substantially crystalline, substantially monocrystalline,
polycrystalline, amorphous, or a combination thereof. In one
aspect, each of the three dimensions of the nanostructure has a
dimension of less than about 500 nm, e.g., less than about 200 nm,
less than about 100 nm, less than about 50 nm, or even less than
about 20 nm.
[0052] An "aspect ratio" is the length of a first axis of a
nanostructure divided by the average of the lengths of the second
and third axes of the nanostructure, where the second and third
axes are the two axes whose lengths are most nearly equal each
other. For example, the aspect ratio for a perfect rod would be the
length of its long axis divided by the diameter of a cross-section
perpendicular to (normal to) the long axis.
[0053] As used herein, the "diameter" of a nanostructure refers to
the diameter of a cross-section normal to a first axis of the
nanostructure, where the first axis has the greatest difference in
length with respect to the second and third axes (the second and
third axes are the two axes whose lengths most nearly equal each
other). The first axis is not necessarily the longest axis of the
nanostructure; e.g., for a disk-shaped nanostructure, the
cross-section would be a substantially circular cross-section
normal to the short longitudinal axis of the disk. Where the
cross-section is not circular, the diameter is the average of the
major and minor axes of that cross-section. For an elongated or
high aspect ratio nanostructure, such as a nanowire, the diameter
is measured across a cross-section perpendicular to the longest
axis of the nanowire. For a spherical nanostructure, the diameter
is measured from one side to the other through the center of the
sphere.
[0054] The terms "crystalline" or "substantially crystalline," when
used with respect to nanostructures, refer to the fact that the
nanostructures typically exhibit long-range ordering across one or
more dimensions of the structure. It will be understood by one of
skill in the art that the term "long range ordering" will depend on
the absolute size of the specific nanostructures, as ordering for a
single crystal cannot extend beyond the boundaries of the crystal.
In this case, "long-range ordering" will mean substantial order
across at least the majority of the dimension of the nanostructure.
In some instances, a nanostructure can bear an oxide or other
coating, or can be comprised of a core and at least one shell. In
such instances it will be appreciated that the oxide, shell(s), or
other coating need not exhibit such ordering (e.g. it can be
amorphous, polycrystalline, or otherwise). In such instances, the
phrase "crystalline," "substantially crystalline," "substantially
monocrystalline," or "monocrystalline" refers to the central core
of the nanostructure (excluding the coating layers or shells). The
terms "crystalline" or "substantially crystalline" as used herein
are intended to also encompass structures comprising various
defects, stacking faults, atomic substitutions, and the like, as
long as the structure exhibits substantial long range ordering
(e.g., order over at least about 80% of the length of at least one
axis of the nanostructure or its core). In addition, it will be
appreciated that the interface between a core and the outside of a
nanostructure or between a core and an adjacent shell or between a
shell and a second adjacent shell may contain non-crystalline
regions and may even be amorphous. This does not prevent the
nanostructure from being crystalline or substantially crystalline
as defined herein.
[0055] The term "monocrystalline" when used with respect to a
nanostructure indicates that the nanostructure is substantially
crystalline and comprises substantially a single crystal. When used
with respect to a nanostructure heterostructure comprising a core
and one or more shells, "monocrystalline" indicates that the core
is substantially crystalline and comprises substantially a single
crystal.
[0056] A "nanocrystal" is a nanostructure that is substantially
monocrystalline. A nanocrystal thus has at least one region or
characteristic dimension with a dimension of less than about 500
nm, e.g., less than about 200 nm, less than about 100 nm, less than
about 50 nm, or even less than about 20 nm. The term "nanocrystal"
is intended to encompass substantially monocrystalline
nanostructures comprising various defects, stacking faults, atomic
substitutions, and the like, as well as substantially
monocrystalline nanostructures without such defects, faults, or
substitutions. In the case of nanocrystal heterostructures
comprising a core and one or more shells, the core of the
nanocrystal is typically substantially monocrystalline, but the
shell(s) need not be. In one aspect, each of the three dimensions
of the nanocrystal has a dimension of less than about 500 nm, e.g.,
less than about 200 nm, less than about 100 nm, less than about 50
nm, or even less than about 20 nm. Examples of nanocrystals
include, but are not limited to, substantially spherical
nanocrystals, branched nanocrystals, and substantially
monocrystalline nanowires, nanorods, nanodots, quantum dots,
nanotetrapods, tripods, bipods, and branched tetrapods (e.g.,
inorganic dendrimers).
[0057] The term "heterostructure" when used with reference to
nanostructures refers to nanostructures characterized by at least
two different and/or distinguishable material types. Typically, one
region of the nanostructure comprises a first material type, while
a second region of the nanostructure comprises a second material
type. In certain embodiments, the nanostructure comprises a core of
a first material and at least one shell of a second (or third etc.)
material, where the different material types are distributed
radially about the long axis of a nanowire, a long axis of an arm
of a branched nanowire, or the center of a nanocrystal, for
example. (A shell can but need not completely cover the adjacent
materials to be considered a shell or for the nanostructure to be
considered a heterostructure; for example, a nanocrystal
characterized by a core of one material covered with small islands
of a second material is a heterostructure.) In other embodiments,
the different material types are distributed at different locations
within the nanostructure; e.g., along the major (long) axis of a
nanowire or along a long axis of arm of a branched nanowire.
Different regions within a heterostructure can comprise entirely
different materials, or the different regions can comprise a base
material (e.g., silicon) having different dopants or different
concentrations of the same dopant.
[0058] A "nanoparticle" is a nanostructure in which each dimension
(e.g., each of the nanostructure's three dimensions) is less than
about 500 nm, e.g., less than about 200 nm, less than about 100 nm,
less than about 50 nm, or even less than about 20 nm. Nanoparticles
can be of any shape, and include, for example, nanocrystals,
substantially spherical particles (having an aspect ratio of about
0.8 to about 1.2), and irregularly shaped particles. Nanoparticles
optionally have an aspect ratio less than about 1.5. Nanoparticles
can be amorphous, crystalline, monocrystalline, partially
crystalline, polycrystalline, or otherwise. Nanoparticles can be
substantially homogeneous in material properties, or in certain
embodiments can be heterogeneous (e.g., heterostructures).
Nanoparticles can be fabricated from essentially any convenient
material or materials, e.g., the nanoparticles can comprise "pure"
materials, substantially pure materials, doped materials and the
like.
[0059] A "nanowire" is a nanostructure that has one principle axis
that is longer than the other two principle axes. Consequently, the
nanowire has an aspect ratio greater than one; nanowires of this
invention typically have an aspect ratio greater than about 1.5 or
greater than about 2. Short nanowires, sometimes referred to as
nanorods, typically have an aspect ratio between about 1.5 and
about 10. Longer nanowires have an aspect ratio greater than about
10, greater than about 20, greater than about 50, or greater than
about 100, or even greater than about 10,000. The diameter of a
nanowire is typically less than about 500 nm, preferably less than
about 200 nm, more preferably less than about 150 nm, and most
preferably less than about 100 nm, about 50 nm, or about 25 nm, or
even less than about 10 nm or about 5 nm. The nanowires of this
invention can be substantially homogeneous in material properties,
or in certain embodiments can be heterogeneous (e.g., nanowire
heterostructures). The nanowires can be fabricated from essentially
any convenient material or materials. The nanowires can comprise
"pure" materials, substantially pure materials, doped materials and
the like, and can include insulators, conductors, and
semiconductors. Nanowires are typically substantially crystalline
and/or substantially monocrystalline, but can be, e.g.,
polycrystalline or amorphous. In some instances, a nanowire can
bear an oxide or other coating, or can be comprised of a core and
at least one shell. In such instances it will be appreciated that
the oxide, shell(s), or other coating need not exhibit such
ordering (e.g. it can be amorphous, polycrystalline, or otherwise).
Nanowires can have a variable diameter or can have a substantially
uniform diameter, that is, a diameter that shows a variance less
than about 20% (e.g., less than about 10%, less than about 5%, or
less than about 1%) over the region of greatest variability and
over a linear dimension of at least 5 nm (e.g., at least 10 nm, at
least 20 nm, or at least 50 nm). Typically the diameter is
evaluated away from the ends of the nanowire (e.g., over the
central 20%, 40%, 50%, or 80% of the nanowire). A nanowire can be
straight or can be, e.g., curved or bent, over the entire length of
its long axis or a portion thereof. In certain embodiments, a
nanowire or a portion thereof can exhibit two- or three-dimensional
quantum confinement. Nanowires according to this invention can
expressly exclude carbon nanotubes, and, in certain embodiments,
exclude "whiskers" or "nanowhiskers", particularly whiskers having
a diameter greater than 100 nm, or greater than about 200 nm.
[0060] A "substantially spherical nanoparticle" is a nanoparticle
with an aspect ratio between about 0.8 and about 1.2. Similarly, a
"substantially spherical nanocrystal" is a nanocrystal with an
aspect ratio between about 0.8 and about 1.2.
[0061] A "catalyst particle" or "nanostructure catalyst" is a
material that facilitates the formation or growth of a
nanostructure. The term is used herein as it is commonly used in
the art relevant to nanostructure growth; thus, use of the word
"catalyst" does not necessarily imply that the chemical composition
of the catalyst particle as initially supplied in a nanostructure
growth reaction is identical to that involved in the active growth
process of the nanostructure and/or recovered when growth is
halted. For example, when gold nanoparticles are used as catalyst
particles for silicon nanowire growth, particles of elemental gold
are disposed on a substrate and elemental gold is present at the
tip of the nanowire after synthesis, though during synthesis the
gold exists as a eutectic phase with silicon. As a contrasting
example, without limitation to any particular mechanism, when
copper nanoparticles are used for VLS or VSS nanowire growth,
particles of elemental copper are disposed on a substrate, and
copper silicide may be present at the tip of the nanowire during
and after synthesis. As yet another example, again without
limitation to any particular mechanism, when copper oxide
nanoparticles are used as catalyst particles for silicon nanowire
growth, copper oxide particles are disposed on a substrate, but
they may be reduced to elemental copper in a reducing atmosphere
employed for nanowire growth and copper silicide may be present at
the tip of the nanowire during and after nanowire synthesis. Both
situations in which the catalyst material maintains the identical
chemical composition and situations in which the catalyst material
changes in chemical composition are explicitly included by used of
the terms "catalyst particle" or "nanostructure catalyst" herein.
Catalyst particles are typically nanoparticles, particularly
discrete nanoparticles. Catalyst particles are distinct from
precursors employed during nanostructure growth, in that
precursors, in contrast to the catalyst particles, serve as a
source for at least one type of atom that is incorporated
throughout the nanostructure (or throughout a core, shell, or other
region of a nanostructure heterostructure).
[0062] A "compound" or "chemical compound" is a chemical substance
consisting of two or more different chemical elements and having a
unique and defined chemical structure, including, e.g., molecular
compounds held together by covalent bonds, salts held together by
ionic bonds, intermetallic compounds held together by metallic
bonds, and complexes held together by coordinate covalent
bonds.
[0063] An "alloy" is a metallic solid solution (complete or
partial) composed of two or more elements. A complete solid
solution alloy has a single solid phase microstructure, while a
partial solution alloy has two or more phases that may or may not
be homogeneous in distribution.
[0064] A "porous" substrate contains pores or voids. In certain
embodiments, a porous substrate can be an array or population of
particles, e.g., a random close pack particle population or a
dispersed particle population. The particles can be of essentially
any desired size and/or shape, e.g., spherical, elongated,
oval/oblong, plate-like (e.g., plates, flakes, or sheets), or the
like. The individual particles can themselves be nonporous or can
be porous (e.g., include a capillary network through their
structure). When employed for nanostructure growth, the particles
can be but typically are not cross-linked. In other embodiments, a
porous substrate can be a mesh or fabric.
[0065] A "carbon-based substrate" refers to a substrate that
comprises at least about 50% carbon by mass. Suitably, a
carbon-based substrate comprises at least about 60% carbon, 70%
carbon, 80% carbon, 90% carbon, 95% carbon, or about 100% carbon by
mass, including 100% carbon. Exemplary carbon-based substrates that
can be used in the practice of the present invention include, but
are not limited to, carbon powder, such as carbon black, fullerene
soot, desulfurized carbon black, graphite, graphite powder,
graphene, graphene powder, or graphite foil. As used throughout,
"carbon black" refers to the material produced by the incomplete
combustion of petroleum products. Carbon black is a form of
amorphous carbon that has an extremely high surface area to volume
ratio. "Graphene" refers to a single atomic layer of carbon formed
as a sheet, and can be prepared as graphene powders. See, e.g.,
U.S. Pat. Nos. 5,677,082, 6,303,266 and 6,479,030, the disclosures
of each of which are incorporated by reference herein in their
entireties. Carbon-based substrates specifically exclude metallic
materials, such as steel, including stainless steel. Carbon-based
substrates can be in the form of sheets or separate particles, as
well as cross-linked structures.
[0066] Unless clearly indicated otherwise, ranges listed herein are
inclusive.
[0067] A variety of additional terms are defined or otherwise
characterized herein.
DETAILED DESCRIPTION
[0068] Traditional batteries, including lithium ion batteries,
comprise an anode, an electrolyte, a cathode, and typically a
separator. The anode of most commercially available lithium ion
batteries is copper foil coated with a mixture of graphite powder
and a polymer blend. The capacity of these materials is limited,
however. There is therefore need for improved anode materials with
greater storage capacity.
[0069] Silicon has a high theoretical specific capacity for lithium
(Li) storage (approximately 4200 mAh/g). However, silicon also
experiences a large volume change on lithiation or delithiation
that renders bulk silicon impractical for use in battery active
materials. Incorporation of silicon nanowires into anodes can
minimize the mechanical stress associated with lithium ion
insertion and extraction. The use of silicon nanowires in anodes
also provides very high silicon surface area and thus high charging
rates. For additional information on incorporation of silicon into
battery anodes, see, e.g., U.S. patent application publication no.
2010/0297502 by Zhu et al. entitled "Nanostructured materials for
battery applications" and references therein, each of which is
incorporated by reference herein in its entirety. Chen et al.
(2011) "Hybrid silicon-carbon nanostructured composites as superior
anodes for lithium ion batteries" Nano Res. 4(3):290-296, Cui et
al. (2009) "Carbon-silicon core-shell nanowires as high capacity
electrode for lithium ion batteries" Nano Letters 9(9)3370-3374,
Chen et al. (2010) "Silicon nanowires with and without carbon
coating as anode materials for lithium-ion batteries" J Solid State
Electrochem 14:1829-1834, and Chan et al. (2010) "Solution-grown
silicon nanowires for lithium-ion battery anodes" ACS Nano
4(3):1443-1450.
[0070] Widespread adoption of lithium ion batteries including
silicon nanowire-based anodes, however, requires large scale
synthesis of silicon nanowires. Currently, silicon nanowires are
typically grown using gold catalyst particles, for example, in a
vapor-liquid-solid (VLS), chemical vapor deposition (CVD) process
in which a feed gas (e.g., silane) is used as the source material.
Gold catalyst on a heated solid substrate is exposed to the feed
gas, liquifies, and absorbs the Si vapor to supersaturation levels.
Nanostructure growth occurs at the liquid-solid interface. See,
e.g., U.S. Pat. No 7,301,199 to Lieber et al. entitled "Nanoscale
wires and related devices," U.S. Pat. No. 7,211,464 to Lieber et
al. entitled "Doped elongated semiconductors, growing such
semiconductors, devices including such semiconductors and
fabricating such devices," Cui et al. (2001) "Diameter-controlled
synthesis of single-crystal silicon nanowires" Appl. Phys. Lett.
78, 2214-2216, and Morales et al. (1998) "A laser ablation method
for the synthesis of crystalline semiconductor nanowires" Science
279, 208-211.
[0071] However, the cost of gold becomes significant when large
scale synthesis of silicon nanowires is contemplated. Additionally,
the liquid state of Au--Si at the eutectic temperature can cause
uncontrollable deposition of gold-based catalyst material and
subsequent silicon growth at undesired locations such as on the
substrate or the sidewalls of the nanowires. Furthermore, gold is
not compatible with semiconductor processing and is prohibited in
industrial clean rooms, which raises additional difficulties for
gold catalyzed synthesis of nanowires intended for such
applications.
[0072] In one aspect, the present invention overcomes the above
noted difficulties by providing methods of producing nanostructures
(including silicon nanowires) that reduce or even eliminate need
for a gold catalyst. For example, methods for growing silicon
nanowires from core-shell nanoparticles having a gold shell are
provided that reduce the amount of gold required for nanostructure
synthesis, as compared to traditional synthesis techniques using
solid gold nanoparticle catalysts. As another example, methods for
growing silicon nanowires and other nanostructures from
copper-based catalysts are provided that eliminate any need for a
gold catalyst. The methods optionally include growing the
nanostructures on a carbon-based porous substrate suitable for
incorporation into a battery anode. Compositions, battery slurries,
battery anodes, and batteries including nanostructures grown on
such substrates from copper-based catalysts are also described. In
addition, methods for production of nanoparticles including copper
or a copper compound and suitable for use as nanostructure
catalysts are provided.
Nanostructure Growth Using Copper-Based Catalyst Materials
[0073] Although growth of silicon nanowires from copper-based
catalysts has been described in U.S. Pat. No. 7,825,036 to Yao et
al. entitled "Method of synthesizing silicon wires" and Renard et
al. (2010) "Catalyst preparation for CMOS-compatible silicon
nanowire synthesis" Nature Nanotech 4:654-657, these methods
produce nanowires on planar solid substrates not suitable for use
as a battery active material or amenable to scaling up for
production of large quantities of nanowires. In contrast, one
aspect of the present invention provides methods for growth of
nanostructures (including but not limited to silicon nanowires) on
porous or particulate substrates, including substrates that are
suitable for use in batteries and/or that facilitate large-scale
nanostructure synthesis.
[0074] Thus, one general class of embodiments provides methods for
producing nanostructures. In the methods, a porous substrate having
catalyst particles disposed thereon is provided, and the
nanostructures are grown from the catalyst particles. The catalyst
particles comprise copper, a copper compound, and/or a copper
alloy.
[0075] The porous substrate is optionally a mesh, fabric, e.g., a
woven fabric (e.g., a carbon fabric), or fibrous mat. In preferred
embodiments, the substrate comprises a population of particles,
sheets, fibers (including, e.g., nanofibers), and/or the like.
Thus, exemplary substrates include a plurality of silica particles
(e.g., a silica powder), a plurality of carbon sheets, carbon
powder (a plurality of carbon particles), natural and/or artificial
(synthetic) graphite, natural and/or artificial (synthetic)
graphite particles, graphene, graphene powder (a plurality of
graphene particles), carbon fibers, carbon nanostructures, carbon
nanotubes, and carbon black. For synthesis of nanostructures, e.g.,
silicon nanowires, for use as a battery active material, the
substrate is typically a carbon-based substrate, for example, a
population of graphite particles. Suitable graphite particles are
commercially available, for example, from Hitachi Chemical Co.,
Ltd. (Ibaraki, Japan, e.g., MAG D-13 artificial graphite).
[0076] In embodiments in which the substrate comprises a population
of particles (e.g., graphite particles), the particles can be of
essentially any desired shape, for example, spherical or
substantially spherical, elongated, oval/oblong, plate-like (e.g.,
plates, flakes, or sheets), and/or the like. Similarly, the
substrate particles (e.g., graphite particles) can be of
essentially any size. Optionally, the substrate particles have an
average diameter between about 0.5 .mu.m and about 50 .mu.m, e.g.,
between about 0.5 .mu.m and about 2 .mu.m, between about 2 .mu.m
and about 10 .mu.m, between about 2 .mu.m and about 5 .mu.m,
between about 5 .mu.m and about 50 .mu.m, between about 10 .mu.m
and about 30 .mu.m, between about 10 .mu.m and about 20 .mu.m,
between about 15 .mu.m and about 25 .mu.m, between about 15 .mu.m
and about 20 .mu.m, or about 20 .mu.m. As will be evident, the size
of the substrate particles can be influenced by the application
ultimately desired for the resulting nanostructures. For example,
where silicon nanostructures (e.g., silicon nanowires) are being
synthesized on a population of graphite particles as the substrate,
the graphite particle size is optionally about 10-20 .mu.m (e.g.,
about 15-20 .mu.m) where the graphite particles and silicon
nanostructures are to be incorporated into a battery where high
storage capacity is desired, whereas graphite particle size is
optionally a few .mu.m (e.g., about 5 .mu.m or less) where the
graphite particles and silicon nanostructures are to be
incorporated into a battery capable of delivering high current or
power. For the latter application, spherical graphite particles are
optionally employed to achieve higher particle density.
[0077] The catalyst particles are disposed on the surface of the
substrate. Thus, for example, where the substrate comprises a
population of particles, the catalyst particles are disposed on the
surface of individual substrate particles. Individual substrate
particles can themselves be porous or nonporous. Where porous
particles are employed as the substrate, the catalyst particles are
typically disposed on the outer surface of the substrate particles,
but can additionally or alternatively be disposed on the interior
surface of micropores or channels within the substrate
particles.
[0078] The catalyst particles can be of essentially any shape,
including, but not limited to, spherical or substantially
spherical, plate-like, oval/oblong, cubic, and/or irregular shapes
(e.g., starfish-shaped). Similarly, the catalyst particles can be
of essentially any desired size but are typically nanoparticles.
For example, the catalyst particles optionally have an average
diameter between about 5 nm and about 100 nm, e.g., between about
10 nm and about 100 nm, between about 20 nm and about 50 nm, or
between about 20 nm and about 40 nm. Optionally, the catalyst
particles have an average diameter of about 20 nm. As is known in
the art, the size of the catalyst particles affects the size of the
resulting nanostructures (e.g., the diameter of resulting
nanowires).
[0079] As noted above, the catalyst particles can comprise copper,
a copper compound, and/or a copper alloy. For example, the catalyst
particles can comprise copper oxide, e.g., copper (I) oxide
(cuprous oxide, Cu.sub.2O), copper (II) oxide (cupric oxide, CuO),
Cu.sub.2O.sub.3, Cu.sub.3O.sub.4, or a combination thereof. Thus,
in one class of embodiments, the catalyst particles comprise copper
(I) oxide (Cu.sub.2O), copper (II) oxide (CuO), or a combination
thereof. In one class of embodiments, the catalyst particles
comprise elemental (i.e., pure-phase) copper (Cu), copper (I) oxide
(Cu.sub.2O), copper (II) oxide (CuO), or a combination thereof. In
one class of embodiments, the catalyst particles comprise elemental
copper and are substantially free of copper compounds (e.g., copper
oxide) or alloys, e.g., as determined by x-ray diffraction (XRD)
and/or energy-dispersive X-ray spectroscopy (EDS). In another class
of embodiments, the catalyst particles consist essentially of
copper oxide (e.g., Cu.sub.2O and/or CuO), e.g., as determined by
XRD and/or EDS. In another class of embodiments, the catalyst
particles comprise copper acetate, copper nitrate, or a copper
complex comprising a chelating agent (e.g., copper tartrate or
copper EDTA), preferably a copper (II) compound or complex. In one
class of embodiments, the catalyst particles comprise a Cu--Ni
alloy.
[0080] As noted above, the chemical composition of the catalyst
particle as initially supplied in a nanostructure growth reaction
may not be identical to that involved in the active growth process
of the nanostructure and/or recovered when growth is halted. For
example, when copper oxide nanoparticles are used as catalyst
particles for silicon nanowire growth, copper oxide particles are
disposed on a substrate, but they may be reduced to elemental
copper in a reducing atmosphere employed for VSS nanowire growth
and copper silicide may be present at the tip of the nanowire
during and after such synthesis. As another example, when elemental
copper nanoparticles are used as catalyst particles for silicon
nanowire growth, copper particles are disposed on a substrate, but
they may be oxidized to copper oxide in ambient atmosphere, then
reduced to elemental copper in a reducing atmosphere employed for
VSS nanowire growth, and copper silicide may be present at the tip
of the nanowire during and after such synthesis. As yet another
example, when nanoparticles comprising a copper compound such as
copper acetate, copper nitrate, or a copper complex including a
chelating agent are used as catalyst particles for silicon nanowire
growth, they may decompose to form copper oxide when heated in
ambient atmosphere and then be reduced to elemental copper in a
reducing atmosphere employed for nanowire growth, and copper
silicide may be present at the tip of the nanowire during and after
such synthesis.
[0081] Catalyst particles can be produced and disposed on the
substrate by essentially any convenient techniques, including, but
not limited to, colloidal synthesis followed by deposition,
adsorption of copper ions or complexes, or electroless deposition.
Thus, in one class of embodiments, providing a porous substrate
having catalyst particles disposed thereon comprises synthesizing
colloidal nanoparticles comprising copper and/or a copper compound
and then depositing the nanoparticles on the substrate. For
additional details on colloidal synthesis of copper-based
nanoparticles, see the section entitled "Colloidal Synthesis of
Copper-Based Nanoparticles" hereinbelow. In another class of
embodiments, providing a porous substrate having catalyst particles
disposed thereon comprises synthesizing discrete particles on the
substrate through electroless deposition of copper directly onto
the substrate. For additional details on electroless deposition of
copper-based nanoparticles, see the section entitled "Electroless
Deposition of Copper-Based Nanoparticles" hereinbelow. In another
class of embodiments, providing a porous substrate having catalyst
particles disposed thereon comprises immersing the porous substrate
in a solution comprising copper ions and/or a copper complex,
whereby the copper ions and/or the copper complex are adsorbed on
the surface of the substrate, thereby forming discrete
nanoparticles on the surface of the substrate. For additional
details on production of copper-based nanoparticles via adsorption,
see the section entitled "Formation of Copper-Based Nanoparticles
Through Adsorption" hereinbelow.
[0082] The methods can be used to synthesize essentially any
desired type of nanostructures, including, but not limited to,
nanowires, whiskers or nanowhiskers, nanofibers, nanotubes, tapered
nanowires or spikes, nanodots, nanocrystals, branched
nanostructures having three or more arms (e.g., nanotetrapods), or
a combination of any of these.
[0083] The nanostructures can be produced from any suitable
material, suitably an inorganic material, and more suitably an
inorganic conductive or semiconductive material. Suitable
semiconductor materials include, e.g., group II-VI, group III-V,
group IV-VI, and group IV semiconductors. Suitable semiconductor
materials include, but are not limited to, Si, Ge, Sn, Se, Te, B, C
(including diamond), P, BN, BP, BAs, AlN, AlP, AlAs, AlSb, GaN,
GaP, GaAs, GaSb, InN, InP, InAs, InSb, AlN, AlP, AlAs, AlSb, GaN,
GaP, GaAs, GaSb, ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe,
HgTe, BeS, BeSe, BeTe, MgS, MgSe, GeS, GeSe, GeTe, SnS, SnSe, SnTe,
PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr, CuI, Si.sub.3N.sub.4,
Ge.sub.3N.sub.4, Al.sub.2O.sub.3, (Al, Ga, In).sub.2 (S, Se,
Te).sub.3, Al.sub.2CO.sub.3 and an appropriate combination of two
or more such semiconductors.
[0084] In one aspect, for example, where the resulting
nanostructures are to be incorporated into a lithium ion battery,
the nanostructures comprise germanium, silicon, or a combination
thereof. In embodiments in which the nanostructures comprise
silicon, the nanostructures can comprise, e.g., monocrystalline
silicon, polycrystalline silicon, amorphous silicon, or a
combination thereof. For example, the nanostructures can comprise
about 20-100% monocrystalline silicon, about 0-50% polycrystalline
silicon, and/or about 0-50% amorphous silicon. In one class of
embodiments, the nanostructures comprise 20-100% (e.g., 50-100%)
monocrystalline silicon and 0-50% amorphous silicon. In one class
of embodiments, the nanostructures comprise 20-100% (e.g., 50-100%)
monocrystalline silicon and 0-50% polycrystalline silicon. The
percentage of monocrystalline, polycrystalline, and/or amorphous
silicon can be measured for the resulting nanostructures as a group
or individually. Individual silicon nanostructures (e.g.,
nanowires) can be a combination of crystalline, polycrystalline and
amorphous material as detected by transmission electron microscopy
(TEM). For example, nanowires can be completely monocrystalline,
can have a monocrystalline core and a polycrystalline shell, can
have a monocrystalline core and an amorphous or microcrystalline
shell (where the grain structure is not visible within the
resolution of TEM), or can have a monocrystalline core and a shell
that transitions from polycrystalline to amorphous (from the core
to the outside of the nanostructure). Thus, in one class of
embodiments, the nanostructures comprise a monocrystalline core and
a shell layer, wherein the shell layer comprises amorphous silicon,
polycrystalline silicon, or a combination thereof.
[0085] The nanostructures optionally include a coating. For
example, silicon nanostructures optionally bear a silicon oxide
coating. As described in U.S. patent application publication no.
2010/0297502 by Zhu et al. entitled "Nanostructured materials for
battery applications," a carbon coating can be applied to the
silicon nanostructures, e.g., where the nanostructures are intended
for incorporation into a battery anode. The nanostructures
optionally have a polymer coating. See also, e.g., U.S. Pat. No.
7,842,432 to Niu et al. entitled "Nanowire structures comprising
carbon" and U.S. patent application publication no. 2011/0008707 by
Muraoka et al. entitled "Catalyst layer for fuel cell membrane
electrode assembly, fuel cell membrane electrode assembly using the
catalyst layer, fuel cell, and method for producing the catalyst
layer."
[0086] In one aspect, the nanostructures are silicon nanowires.
Nanowires produced by the methods can be of essentially any desired
size. For example, the nanowires can have a diameter of about 10 nm
to about 500 nm, or about 20 nm to about 400 nm, about 20 nm to
about 300 nm, about 20 nm to about 200 nm, about 20 nm to about 100
nm, about 30 nm to about 100 nm, or about 40 nm to about 100 nm.
Typically, the nanowires have an average diameter less than about
150 nm, e.g., between about 10 nm and about 100 nm, e.g., between
about 30 nm and about 50 nm, e.g., between about 40 nm and about 45
nm. The nanowires are optionally less than about 100 mm in length,
e.g., less than about 10 .mu.m, about 100 nm to about 100 .mu.m, or
about 1 .mu.m to about 75 .mu.m, about 1 .mu.m to about 50 .mu.m,
or about 1 mm to about 20 .mu.m in length. The aspect ratios of the
nanowires are optionally up to about 2000:1 or about 1000:1. For
example, the nanowires can have a diameter of about 20 nm to about
200 nm and a length of about 0.1 .mu.m to about 50 mm.
[0087] The nanostructures can be synthesized using essentially any
convenient technique. As one example, a vapor-liquid-solid (VLS)
technique such as that described above for gold catalyst particles
can be employed with the copper-based catalyst. VLS techniques
employing copper catalysts typically require high temperatures,
however (e.g., above 800.degree. C. for silicon nanowires).
Vapor-solid-solid (VSS) techniques in which the copper-based
catalyst remains in the solid phase are typically more convenient
since they can be performed at lower temperatures (e.g., about
500.degree. C. for silicon nanowires). VSS and VLS techniques are
known in the art; see, e.g., U.S. patent application publication
no. 2011/0039690 by Niu et al. entitled "Porous substrates,
articles, systems and compositions comprising nanofibers and
methods of their use and production," U.S. Pat. No. 7,825,036 to
Yao et al. entitled "Method of synthesizing silicon wires," Renard
et al. (2010) "Catalyst preparation for CMOS-compatible silicon
nanowire synthesis" Nature Nanotech 4:654-657, U.S. Pat. No.
7,776,760 to Taylor entitled "Systems and methods for nanowire
growth," U.S. Pat. No. 7,301,199 to Lieber et al. entitled
"Nanoscale wires and related devices," U.S. Pat. No. 7,211,464 to
Lieber et al. entitled "Doped elongated semiconductors, growing
such semiconductors, devices including such semiconductors and
fabricating such devices," Cui et al. (2001) "Diameter-controlled
synthesis of single-crystal silicon nanowires" Appl. Phys. Lett.
78, 2214-2216, Morales et al. (1998) "A laser ablation method for
the synthesis of crystalline semiconductor nanowires" Science 279,
208-211, and Qian et al. (2010) "Synthesis of
germanium/multi-walled carbon nanotube core-sheath structures via
chemical vapor deposition" in N. Lupu (Ed.), Nanowires Science and
Technology (pp. 113-130) Croatia, INTECH. See also Example 1
hereinbelow. For synthesis of silicon nanostructures (e.g., silicon
nanowires), the crystallinity of the resulting nanostructures can
be controlled, e.g., by controlling the growth temperature,
precursors, and/or other reaction conditions that are employed. A
chlorinated silane precursor or an etchant gas such as HCl can be
employed to prevent undesired deposition of silicon at locations
other than the catalyst (e.g., exposed substrate surfaces or the
sidewall of the reaction chamber) and/or tapering of the nanowires
due to dripping of molten catalyst down the growing nanowire
leading to growth on the sidewall of the nanowire (which can also
result in formation of an amorphous or polycrystalline shell on the
nanowire); see, e.g., U.S. Pat. Nos. 7,776,760 and 7,951,422. These
problems are greatly reduced by use of a solid copper-based
catalyst instead of a liquid gold catalyst, so use of a
copper-based catalyst can reduce or eliminate need for inclusion of
an etchant (or use of a chlorinated silane precursor) in the
nanostructure synthesis process.
[0088] Additional information on nanostructure synthesis using
various techniques is readily available in the art. See, e.g., U.S.
Pat. No. 7,105,428 to Pan et al. entitled "Systems and methods for
nanowire growth and harvesting," U.S. Pat. No. 7,067,867 to Duan et
al. entitled "Large-area nonenabled macroelectronic substrates and
uses therefor," U.S. Pat. No. 7,951,422 to Pan et al. entitled
"Methods for oriented growth of nanowires on patterned substrates,"
U.S. Pat. No. 7,569,941 to Majumdar et al. entitled "Methods of
fabricating nanostructures and nanowires and devices fabricated
therefrom," U.S. Pat. No. 6,962,823 to Empedocles et al. entitled
"Methods of making, positioning and orienting nanostructures,
nanostructure arrays and nanostructure devices," U.S. patent
application Ser. No. 12/824,485 by Dubrow et al. entitled
"Apparatus and methods for high density nanowire growth," Gudiksen
et al (2000) "Diameter-selective synthesis of semiconductor
nanowires" J. Am. Chem. Soc. 122, 8801-8802; Gudiksen et al. (2001)
"Synthetic control of the diameter and length of single crystal
semiconductor nanowires" J. Phys. Chem. B 105,4062-4064; Duan et
al. (2000) "General synthesis of compound semiconductor nanowires"
Adv. Mater. 12, 298-302; Cui et al. (2000) "Doping and electrical
transport in silicon nanowires" J. Phys. Chem. B 104, 5213-5216;
Peng et al. (2000) "Shape control of CdSe nanocrystals" Nature 404,
59-61; Puntes et al. (2001) "Colloidal nanocrystal shape and size
control: The case of cobalt" Science 291, 2115-2117; U.S. Pat. No.
6,306,736 to Alivisatos et al. (Oct. 23, 2001) entitled "Process
for forming shaped group III-V semiconductor nanocrystals, and
product formed using process"; U.S. Pat. No. 6,225,198 to
Alivisatos et al. (May 1, 2001) entitled "Process for forming
shaped group II-VI semiconductor nanocrystals, and product formed
using process"; U.S. Pat. No. 6,036,774 to Lieber et al. (Mar. 14,
2000) entitled "Method of producing metal oxide nanorods"; U.S.
Pat. No. 5,897,945 to Lieber et al. (Apr. 27, 1999) entitled "Metal
oxide nanorods"; U.S. Pat. No. 5,997,832 to Lieber et al. (Dec. 7,
1999) "Preparation of carbide nanorods"; Urbau et al. (2002)
"Synthesis of single-crystalline perovskite nanowires composed of
barium titanate and strontium titanate" J. Am. Chem. Soc., 124,
1186; and Yun et al. (2002) "Ferroelectric Properties of Individual
Barium Titanate Nanowires Investigated by Scanned Probe Microscopy"
Nanoletters 2, 447.
[0089] Synthesis of core-shell nanostructure heterostructures,
namely nanocrystal and nanowire core-shell heterostructures, are
described in, e.g., Peng et al. (1997) "Epitaxial growth of highly
luminescent CdSe/CdS core/shell nanocrystals with photostability
and electronic accessibility" J. Am. Chem. Soc. 119, 7019-7029;
Dabbousi et al. (1997) "(CdSe)ZnS core-shell quantum dots:
Synthesis and characterization of a size series of highly
luminescent nanocrystallites" J. Phys. Chem. B 101, 9463-9475;
Manna et al. (2002) "Epitaxial growth and photochemical annealing
of graded CdS/ZnS shells on colloidal CdSe nanorods" J. Am. Chem.
Soc. 124, 7136-7145; and Cao et al. (2000) "Growth and properties
of semiconductor core/shell nanocrystals with InAs cores" J. Am.
Chem. Soc. 122, 9692-9702. Similar approaches can be applied to
growth of other core-shell nanostructures. Growth of nanowire
heterostructures in which the different materials are distributed
at different locations along the long axis of the nanowire is
described in, e.g., Gudiksen et al. (2002) "Growth of nanowire
superlattice structures for nanoscale photonics and electronics"
Nature 415, 617-620; Bjork et al. (2002) "One-dimensional
steeplechase for electrons realized" Nano Letters 2, 86-90; Wu et
al. (2002) "Block-by-block growth of single-crystalline Si/SiGe
superlattice nanowires" Nano Letters 2, 83-86; and US patent
application publication no. 2004/0026684 to Empedocles entitled
"Nanowire heterostructures for encoding information." Similar
approaches can be applied to growth of other heterostructures.
[0090] In embodiments in which the substrate comprises a population
of particles (e.g., graphite or silica particles), the substrate
particles with catalyst particles disposed thereon are typically
loaded into a reaction vessel in which nanostructure synthesis is
subsequently performed. For example, the substrate particles can be
loaded into a quartz tube or cup with a porous frit (e.g., a quartz
frit) to retain the particles, e.g., as gas flows through the
vessel during a CVD (e.g., VLS or VSS) nanostructure synthesis
reaction.
[0091] The substrate particles can form a packed bed in the
reaction vessel. Without limitation to any particular mechanism,
conversion of reactant (e.g., a source or precursor gas) depends on
the relative reaction and gas flow rates. Minimal variation in
reactant concentration throughout the bed can be achieved, or high
total conversion can be achieved. In the case of high total
conversion, the amount of nanostructures grown on the substrate
particles typically varies from the entrance to the exit of the
packed bed due to depletion of the source gas. This effect can be
mitigated, if desired, e.g., by flowing the reactant gas in both
directions through the vessel or by mixing the substrate particles
during the growth process.
[0092] Where mixing of the substrate particles is desired, the
reaction vessel can contain a mechanical stirrer or mixer that acts
to redistribute the substrate particles in the vessel over the
course of the nanostructure synthesis reaction. Convection of
particles within the bed can allow each particle to experience
similar growth conditions (e.g., temperature and reactant
concentration) on average, particularly when recirculation of
particles within the bed is faster than the growth rate of the
nanostructures. For example, the reaction vessel can include a
helical ribbon or a rotating impeller blade, e.g., in a vertical or
horizontal reaction vessel. As another example, the reaction vessel
can be horizontal and made to rotate; rotation of the vessel drags
the substrate particles up the vessel walls, resulting in mixing. A
component of the vessel is optionally fixed (i.e., not rotating),
for example, a tube in the center of the vessel for injection of
gases. Other components are optionally fixed to the static
component, for example, a scraper to prevent sticking of material
to the vessel walls (e.g., a thin band or wire comformal to the
vessel walls) or an array of rigid pins. The reaction vessel can
include two linear arrays of regularly spaced rigid pins, one fixed
to the rotating wall and the other to the static inlet tube, with
the pins normal to the tube walls. The moving and fixed arrays of
pins are offset in an interdigitated fashion, so that they do not
collide but instead push any aggregates of substrate particles
between the pins, breaking up and limiting aggregate size. As
additional examples, the substrate particles can be fluidized by
ultrasonic or mechanical shaking of the bed instead of or in
addition to by mechanical stirring.
[0093] It is worth noting that bed volume typically increases with
increasing gas flow rate. It is also worth noting that at very low
pressure, for example, less than about 500 mtorr, fluidization of
substrate particles is impeded. Growth pressure ranges above about
500 mtorr (e.g., medium-low vacuum, above 200 to 400 torr, to
near-atmospheric, atmospheric, or above-atmospheric pressure) are
therefore generally preferred for nanostructure growth in a mixed
bed.
[0094] The methods can be employed for production of nanostructures
for use in any of a variety of different applications. For example,
as noted above, the nanostructures and optionally the substrate on
which the nanostructures were grown can be incorporated into a
battery, battery anode, and/or battery slurry. In one class of
embodiments, the nanostructures and substrate are incorporated into
the anode electrode of a lithium ion battery.
[0095] A lithium ion battery typically includes an anode, an
electrolyte (e.g., an electrolyte solution), and a cathode. A
separator (e.g., a polymer membrane) is typically placed between
the anode and the cathode in embodiments in which the electrolyte
is, e.g., a liquid or gel. In embodiments where a solid-state
electrolyte is employed, a separator is typically not included. The
anode, electrolyte, cathode, and separator (if present) are encased
in a housing.
[0096] Suitable materials for the housing, cathode, electrolyte,
and separator are known in the art. See, e.g., U.S. patent
application publication no. 2010/0297502. For example, the housing
can be a metal, polymer, ceramic, composite, or like material, or
can include a combination of such materials (e.g., a laminate of
metallic and polymer layers). The cathode can comprise any suitable
material known for use as a battery cathode, including, but not
limited to, lithium-containing materials such as LiCoO.sub.2,
LiFePO.sub.4, LiMnO.sub.2, LiMnO.sub.4,
LiNiCoAlO/LiNiCoMnO.sup.+LiMn.sub.2O.sub.4, LiCoFePO.sub.4 and
LiNiO.sub.2. The electrolyte can comprise a solid-state electrolyte
(e.g., an alkali metal salt, e.g., a lithium salt, mixed with an
ionically conducting material) or an electrolyte solution (e.g., an
alkali metal salt, e.g., a lithium salt, e.g., LiPF.sub.6,
dissolved in a solvent, e.g. an organic solvent, e.g., diethyl
carbonate, ethylene carbonate, ethyl methyl carbonate, or a
combination thereof). The separator can be a microporous polymer
material having good ionic conductivity and sufficiently low
electronic conductivity, e.g., PVDF, polypyrrole, polythiaphene,
polyethylene oxide, polyacrylonitrile, poly(ethylene succinate),
polypropylene, poly (.beta.-propiolactone), or a sulfonated
fluoropolymer such as NAFION.RTM..
[0097] In one class of embodiments, after nanostructure synthesis,
the nanostructures and the substrate are incorporated into a
battery slurry, e.g., by mixing the substrate bearing the
nanostructures with a polymer binder and a solvent (e.g., water or
an organic solvent). Suitable binders (e.g., conductive polymers)
and solvents are known in the art. Examples include, but are not
limited to, polyvinylidene difluoride (PVDF) as the binder with
N-methyl-2-pyrrolidone (NMP) as the solvent or carboxymethyl
cellulose (CMC) as the binder with water as the solvent. The
battery slurry (which can also be referred to as an active material
slurry) is coated on a current collector, e.g., copper foil.
Evaporation of the solvent leaves the active materials (the
nanostructures and the substrate) and the polymer binder coating
the current collector. This assembly can then be employed as a
battery anode, e.g., after insertion into a suitable housing along
with a cathode, electrolyte, and optionally a separator placed in
the electrolyte between the anode and cathode.
[0098] In one class of embodiments, the substrate comprises a
population of graphite particles and the nanostructures comprise
silicon nanowires. Optionally, for example, in embodiments in which
the nanowires and graphite particles are to be incorporated into a
battery, slurry, or anode, when nanostructure growth is finished,
silicon comprises between 1% and 50% of the total weight of the
nanostructures and the graphite particles, e.g., between 1% and
40%, between 1% and 30%, or between 2% and 25%. Optionally, silicon
comprises between 2% and 20% of the total weight of the
nanostructures and the graphite particles, or between 6% and 20% of
the total weight of the nanostructures and the graphite particles.
Where the nanowires and graphite are used in a battery, it will be
evident that increasing the silicon content tends to increase the
capacity per gram and that the silicon content is desirably matched
with the specific capacity of the cathode.
Battery Active Materials Synthesized Using Copper-Based
Catalysts
[0099] Compositions produced by or useful in practicing any of the
methods herein are also a feature of the invention. For example,
nanostructures grown from copper-based catalysts on porous
substrates are a feature of the invention.
[0100] Accordingly, one general class of embodiments provides a
composition that includes a porous substrate and a population of
nanostructures attached thereto. A region of each of the
nanostructures is attached to the substrate, and another region of
each nanostructure (generally distal to the first region) comprises
copper, a copper compound, and/or a copper alloy, equivalent to or
derived from the catalyst that was employed in the synthesis
reaction. Attachment of the nanostructures to the substrate on
which they were grown is typically through van der Waals
interactions.
[0101] Thus, in one class of embodiments, the composition includes
a porous substrate and a population of silicon nanowires attached
thereto, wherein one end of a member nanowire is attached to the
substrate and the other end of the member nanowire comprises
copper, a copper compound, and/or a copper alloy.
[0102] Essentially all of the features noted for the embodiments
above apply to these embodiments as well, as relevant; for example,
with respect to type, composition, and size of nanostructures,
composition and configuration of the substrate, catalyst material,
incorporation into a battery slurry, battery anode, or battery,
and/or the like.
[0103] For example, the composition can include nanowires having an
average diameter of about 10 nm to about 500 nm, or about 20 nm to
about 400 nm, about 20 nm to about 300 nm, about 20 nm to about 200
nm, about 20 nm to about 100 nm, about 30 nm to about 100 nm, or
about 40 nm to about 100 nm. Typically, the nanowires have an
average diameter less than about 150 nm, e.g., between about 10 nm
and about 100 nm, e.g., between about 30 nm and about 50 nm, e.g.,
between about 40 nm and about 45 nm. The nanowires are optionally
less than about 100 .mu.m in length, e.g., less than about 10
.mu.m, about 100 nm to about 100 .mu.m, or about 1 .mu.m to about
75 .mu.m, about 1 .mu.m to about 50 .mu.m, or about 1 .mu.m to
about 20 .mu.m in length. The aspect ratios of the nanowires are
optionally up to about 2000:1 or about 1000:1. For example, the
nanowires can have a diameter of about 20 nm to about 200 nm and a
length of about 0.1 .mu.m to about 50 .mu.m. The nanowires can
comprise monocrystalline silicon, polycrystalline silicon,
amorphous silicon, or a combination thereof, e.g., in the relative
amounts detailed above. For example, the nanowires optionally
comprise a monocrystalline core and a shell layer, wherein the
shell layer comprises amorphous silicon, polycrystalline silicon,
or a combination thereof.
[0104] As for the embodiments above, the porous substrate is
optionally a mesh, fabric, e.g., a woven fabric (e.g., a carbon
fabric), or fibrous mat. In preferred embodiments, the substrate
comprises a population of particles, sheets, fibers (including,
e.g., nanofibers), and/or the like. Thus, exemplary substrates
include a plurality of silica particles (e.g., a silica powder), a
plurality of carbon sheets, carbon powder or a plurality of carbon
particles, natural and/or artificial (synthetic) graphite, natural
and/or artificial (synthetic) graphite particles, graphene,
graphene powder or a plurality of graphene particles, carbon
fibers, carbon nanostructures, carbon nanotubes, and carbon black.
Where the nanostructures, e.g., silicon nanowires, are intended for
use as a battery active material, the substrate is typically a
carbon-based substrate, for example, a population of graphite
particles.
[0105] In embodiments in which the substrate comprises a population
of particles (e.g., graphite particles), the particles can be of
essentially any desired shape, for example, spherical or
substantially spherical, elongated, oval/oblong, plate-like (e.g.,
plates, flakes, or sheets), and/or the like. Similarly, the
substrate particles can be of essentially any size. Optionally, the
substrate particles have an average diameter between about 0.5
.mu.m and about 50 .mu.m, e.g., between about 0.5 .mu.m and about 2
.mu.m, between about 2 .mu.m and about 10 .mu.m, between about 2
.mu.m and about 5 .mu.m, between about 5 .mu.m and about 50 .mu.m,
between about 10 .mu.m and about 30 .mu.m, between about 10 .mu.m
and about 20 .mu.m, between about 15 .mu.m and about 25 .mu.m,
between about 15 .mu.m and about 20 .mu.m, or about 20 .mu.m. In
one exemplary class of embodiments, the nanostructures are silicon
nanowires, the substrate is a population of graphite particles, and
the graphite particle size is about 10-20 .mu.m. In another
exemplary class of embodiments, the nanostructures are silicon
nanowires, the substrate is a population of graphite particles, the
graphite particle size is a few .mu.m (e.g., about 2 .mu.m or
less), and the graphite particles are optionally spherical.
[0106] The catalyst-derived material on the distal region of the
nanostructures, e.g., the ends of the member nanowires not attached
to the substrate, can comprise, e.g., elemental copper, copper
oxide, copper silicide, or a combination thereof.
[0107] The composition optionally includes a polymer binder (for
example, any of those noted above) and/or a solvent (e.g., water or
an organic solvent as noted above). In one class of embodiments,
for example, where the composition is included in a battery,
slurry, or anode, the substrate comprises a population of graphite
particles, the nanostructures are silicon nanostructures, and
silicon comprises between 1% and 50% of the total weight of the
nanostructures and the graphite particles, e.g., between 1% and
40%, between 1% and 30%, or between 2% and 25%. Optionally, silicon
comprises between 2% and 20% of the total weight of the
nanostructures and the graphite particles, or between 6% and 20% of
the total weight of the nanostructures and the graphite
particles.
[0108] A battery slurry comprising a composition of the invention
is also a feature of the invention. Thus, one class of embodiments
provides a battery slurry comprising a porous substrate, a
population of silicon nanowires, a polymer binder, and a solvent,
where one end of a member nanowire is attached to the substrate and
the other end of the member nanowire comprises copper, a copper
compound, and/or a copper alloy. Similarly, a battery anode
comprising a composition of the invention is also a feature of the
invention, as is a battery comprising a composition of the
invention. Thus, one class of embodiments provides a battery (e.g.,
a lithium ion battery) that includes an anode comprising a porous
substrate and a population of silicon nanowires attached thereto,
wherein one end of a member nanowire is attached to the substrate
and the other end of the member nanowire comprises copper, a copper
compound, and/or a copper alloy. A polymer binder is typically also
included in the anode, and the battery typically also includes a
cathode, an electrolyte, and an optional separator, encased in a
housing. Suitable binders, cathode materials, electrolytes, housing
materials, and the like have been noted hereinabove.
Colloidal Synthesis of Copper-Based Nanoparticles
[0109] Copper-containing nanoparticles are of interest in a wide
variety of applications. For example, cuprous oxide (Cu.sub.2O) is
a p-type semiconductor with potential applications in solar energy
conversion, catalysis (e.g., CO oxidation and photoactivated water
splitting into H.sub.2 and O.sub.2), and gas sensing and as an
anode material for lithium ion batteries. As noted above, cuprous
oxide and other copper-based nanoparticles are also of interest as
catalysts for nanostructure synthesis.
[0110] Nanoparticles containing copper, copper alloy, and/or a
copper compound can be conveniently prepared using colloidal
synthesis techniques. As used herein, synthesis of colloidal
nanoparticles refers to production of a colloid mixture that
includes a solvent and nanoparticles, where the particles are
dispersed evenly throughout the solvent. "Colloidal nanoparticles"
refers to nanoparticles produced via colloidal synthesis, even if
the nanoparticles are subsequently removed from the solvent, for
example, by deposition on a solid substrate.
[0111] Cu.sub.2O nanoparticles can be synthesized from a modified
Fehling's solution, where Cu.sup.2+, protected by a capping agent,
is reduced to Cu.sub.2O in an alkaline aqueous solution.
Nanoparticles that precipitate out of the solution when synthesized
via this route can be resuspended to form a colloid if appropriate
ligands are attached to the nanoparticles. For example, Cu.sub.2O
nanoparticles can be synthesized as a stable Cu.sub.2O colloid in
water (i.e., a hydrosol), using sodium ascorbate or ascorbic acid
to reduce a copper (II) salt to Cu.sub.2O in the presence of
surfactant. Controlling conditions such as pH and concentrations
results in formation of colloidal Cu.sub.2O (stable for at least
several hours), with a narrow particle size distribution and
average particle size varying, e.g., from 15 to 100 nm as
determined by electron microscopy and/or light scattering. In
particular, colloidal Cu.sub.2O, which is stable for a few hours,
has been synthesized from the reduction of copper (II) sulfate
using sodium ascorbate or ascorbic acid in the presence of sodium
dodecyl sulfate (an anionic surfactant), Triton X-100 (a nonionic
surfactant), or cetrimonium chloride (a cationic surfactant). FIG.
1 Panels A and B show colloidal Cu.sub.2O nanoparticles synthesized
in this manner; the average size was 33 nm as measured for 50
particles by electron microscopy, while results from dynamic light
scattering measurements show a strong peak around 50 nm. (In
general, particle size as measured by light scattering tends to be
greater than that measured by electron microscopy, since light
scattering measures the hydrodynamic diameter which depends on
ionic strength of the suspension and surface structure of the
adsorbed surfactant layer.) If the solution pH is too high, the
Cu.sub.2O particles tend to grow excessively, resulting in
precipitates. On the other hand, if the pH is too low, small
Cu.sub.2O particles tend to dissolve in water and the colloid
becomes unstable. When both pH and concentrations are appropriate
and the Cu.sub.2O surfaces are protected by suitable functional
groups, a stable Cu.sub.2O hydrosol exists, with a unique color
ranging from light green to golden yellow. For example, colloidal
Cu.sub.2O, which is stable for at least one hour, may be obtained
in a pH range of 8-11, with up to 5 millimolar copper ions.
[0112] As will be evident, various combinations of copper salt,
reducing agent, capping agent, and surfactant can be employed.
Exemplary reducing agents include, but are not limited to, glucose,
formaldehyde, ascorbic acid, and phosphorous acid. Exemplary
capping agents include, but are not limited to, tartaric acid and
sodium citrate. Suitable surfactants/dispersants include, but are
not limited to, nonionic surfactants (e.g., Triton X-100), cationic
surfactants (e.g., cetrimonium chloride or cetrimonium bromide
(CTAB)), anionic surfactants (e.g., sodium dodecyl sulfate (SDS)),
and polymers such as polyethylene glycol (e.g., molecular weight
600-8000) and polyvinylpyrrolidone (e.g., molecular weight
55,000).
[0113] Copper-containing nanoparticles, including Cu.sub.2O
nanoparticles, can also be synthesized in non-aqueous solutions.
See, e.g., Yin et al. (2005) "Copper oxide nanocrystals" J Am Chem
Soc 127:9506-9511 and Hung et al. (2010) "Room-temperature
formation of hollow Cu.sub.2O nanoparticles" Adv Mater
22:1910-1914.
[0114] Additional information on colloidal synthesis of
nanoparticles is available in the art. See, e.g., Kuo et al. (2007)
"Seed-mediated synthesis of monodispersed Cu.sub.2O nanocubes with
five different size ranges from 40 to 420 nm" Adv. Funct. Mater.
17:3773-3780 and Kooti and Matouri (2010) "Fabrication of nanosized
cuprous oxide using Fehling's solution" Transaction F:
Nanotechnology 17(1):73-78 for synthesis of Cu.sub.2O and Yu et al.
(2009) "Synthesis and characterization of monodispersed copper
colloids in polar solvents" Nanoscale Res Lett. 4:465-470 for
synthesis of elemental copper colloids.
[0115] The colloidal nanoparticles are optionally employed as
catalyst particles for nanostructure synthesis. See, for example,
FIG. 2 Panels A and B, which show colloidal Cu.sub.2O nanoparticles
deposited on a graphite particle substrate and silicon nanowires
grown from the Cu.sub.2O nanoparticles. The resulting nanowires are
similar to nanowires conventionally grown using gold catalyst
particles.
[0116] Thus, one general class of embodiments provides methods for
synthesizing nanostructures. In the methods, colloidal
nanoparticles comprising copper and/or a copper compound are
synthesized and deposited on a substrate, and nanostructures are
grown from the nanoparticles. Exemplary nanostructures and
materials are noted hereinabove and include, but are not limited
to, silicon nanowires.
[0117] Accordingly, in one class of embodiments, colloidal
nanoparticles comprising copper and/or a copper compound are
synthesized. After their synthesis, the nanoparticles are deposited
on a substrate. Nanowires are then grown from the
nanoparticles.
[0118] Suitable substrates include a planar substrate, silicon
wafer, or foil (e.g., a metal foil, e.g., stainless steel foil).
Suitable substrates include nonporous substrates as well as porous
substrates such as those described above, e.g., a mesh, fabric,
e.g., a woven fabric (e.g., a carbon fabric), fibrous mat,
population of particles, sheets, fibers (including, e.g.,
nanofibers), and/or the like. Thus, exemplary substrates include a
plurality of silica particles (e.g., a silica powder), a plurality
of carbon sheets, carbon powder or a plurality of carbon particles,
natural and/or artificial (synthetic) graphite, natural and/or
artificial (synthetic) graphite particles, graphene, graphene
powder or a plurality of graphene particles, carbon fibers, carbon
nanostructures, carbon nanotubes, and carbon black. Where the
nanostructures, e.g., silicon nanowires, are intended for use as a
battery active material, the substrate is typically a carbon-based
substrate, for example, a population of graphite particles.
[0119] In embodiments in which the substrate comprises a population
of particles (e.g., graphite particles), the particles can be of
essentially any desired shape, for example, spherical or
substantially spherical, elongated, oval/oblong, plate-like (e.g.,
plates, flakes, or sheets), and/or the like. Similarly, the
substrate particles can be of essentially any size. Optionally, the
substrate particles have an average diameter between about 0.5
.mu.m and about 50 .mu.m, e.g., between about 0.5 .mu.m and about 2
.mu.m, between about 2 .mu.m and about 10 .mu.m, between about 2
.mu.m and about 5 .mu.m, between about 5 .mu.m and about 50 .mu.m,
between about 10 .mu.m and about 30 .mu.m, between about 10 .mu.m
and about 20 .mu.m, between about 15 .mu.m and about 25 .mu.m,
between about 15 .mu.m and about 20 .mu.m, or about 20 .mu.m. In
one exemplary class of embodiments, the nanostructures are silicon
nanowires, the substrate is a population of graphite particles, and
the graphite particle size is about 10-20 .mu.m. In another
exemplary class of embodiments, the nanostructures are silicon
nanowires, the substrate is a population of graphite particles, the
graphite particle size is a few .mu.m (e.g., about 2 .mu.m or
less), and the graphite particles are optionally spherical.
[0120] The copper compound is optionally copper oxide. In one class
of embodiments, the nanoparticles comprise elemental copper (Cu),
copper (I) oxide (Cu.sub.2O), copper (II) oxide (CuO), or a
combination thereof Optionally, the substrate comprises a
population of graphite particles.
[0121] The shape and size of the nanoparticles can vary, for
example, depending on the diameter desired for the resulting
nanowires. For example, the nanoparticles optionally have an
average diameter between about 5 nm and about 100 nm, e.g., between
about 10 nm and about 100 nm, between about 20 nm and about 50 nm,
or between about 20 nm and about 40 nm. Optionally, the
nanoparticles have an average diameter of about 20 nm. The
nanoparticles can be of essentially any shape, including, but not
limited to, spherical, substantially spherical, or other regular or
irregular shapes.
[0122] Essentially all of the features noted for the embodiments
above apply to these embodiments as well, as relevant; for example,
with respect to nanostructure growth technique (e.g., VLS or VSS),
type, composition, and size of the resulting nanostructures, ratio
of nanostructures to substrate (e.g., silicon to graphite) by
weight, incorporation into a battery slurry, battery anode, or
battery, and/or the like.
[0123] The nanoparticles can be deposited on the substrate using
essentially any convenient technique, for example, spin coating,
spraying, dipping, or soaking. Nanoparticles can be deposited on a
particulate substrate (e.g., graphite particles) by stirring a
mixture of the nanoparticles and substrate particles, e.g., in a
solvent; the substrate particles with the nanoparticles deposited
thereon can then be recovered by filtration and optionally dried to
remove residual solvent. Preferably, the nanoparticles are
deposited on the substrate surface and their phase and/or
morphology is preserved before nanostructure synthesis is commenced
(e.g., by introduction of silane gas into the reaction chamber).
Deposition of the nanoparticles on the substrate is desirably
even.
[0124] The colloid and/or the substrate can be treated or modified
to enhance association of the nanoparticles with the surface of the
substrate. Where the colloid is negatively charged and the
substrate is positively charged (or vice versa), the nanoparticles
generally stick to the surface of the substrate well. If necessary,
however, the charge or other surface characteristics of either or
both the colloid and the surface can be modified. For example,
where elemental copper or copper oxide nanoparticles are deposited
on graphite particles as the substrate, the graphite can be
treated, e.g., with acid, to increase adherence of the
nanoparticles to the graphite. Ligands on the nanoparticles (e.g.,
surfactants) and/or the substrate can be varied.
Electroless Deposition of Copper-Based Nanoparticles
[0125] Electroless plating generally involve immersion of a
substrate in a solution containing a metal salt. Chemical reactions
on the substrate's surface result in plating of the metal out of
solution onto the surface. The reactions occur without use of
external electrical power.
[0126] For example, electroless copper plating can be achieved by
immersing an activated substrate in a bath containing a copper
source (e.g., a copper (II) salt), a chelating agent, and a
reducing agent such as formaldehyde, typically at alkaline pH.
Without limitation to any particular mechanism, deposition of
copper can occur through the following reactions:
Cu[L].sub.x.sup.2++2e.sup.-.fwdarw.Cu+xL Cathodic:
2HCHO+4OH.sup.-.fwdarw.2HCOO.sup.-+H.sub.2+2H.sub.2O+2e.sup.-
Anodic:
2HCHO+4OH.sup.-+Cu[L].sub.x.sup.2+.fwdarw.Cu+2HCOO.sup.-+H.sub.2+2H.sub.-
2O+xL Overall:
The reaction is autocatalytic only in the presence of an activated
surface or when hydrogen is being generated. Deposition rate
depends on, e.g., the copper complexing agent, reducing agent, pH,
bath temperature, and any additives that may be included in the
plating solution (e.g., stabilizer, surfactant, accelerator,
etc.).
[0127] Electroless plating has been used to plate or deposit a thin
layer of copper film on substrates, particularly dielectric
materials such as PCB (plastic circuit board). The process is,
however, not user-friendly, particularly when the substrate is in
the form of fine powders which require filtration to separate the
solid from the liquid effluent. To achieve the desired copper
level, plating parameters (e.g., time of immersion and bath
temperature) must be precisely controlled. In addition, critical
ingredients in the plating bath must be analyzed and replenished to
maintain the process stability. See, e.g., U.S. Pat. No. 4,136,216
to Feldstein entitled "Non-precious metal colloidal dispersions for
electroless metal deposition"; U.S. Pat. No. 4,400,436 to
Breininger et al. entitled "Direct electroless deposition of
cuprous oxide films"; U.S. Pat. No. 4,171,225 to Molenaar et al.
entitled "Electroless copper plating solutions"; Liu et al. (1999)
"Modifications of synthetic graphite for secondary lithium-ion
battery applications" J Power Sources 81-82:187-191; Lu et al.
(2002) "Electrochemical and thermal behavior of copper coated type
MAG-20 natural graphite" Electrochimica Acta 47(10):1601-1606; Yu
(2007) "A novel processing technology of electroless copper plating
on graphite powder" Materials Protection 2007-09; Bindra and White
(1990) "Fundamental aspects of electroless copper plating" in
Mallory and Hajdu (Eds.) Electroless Plating--Fundamentals and
Applications (pp. 289-329) William Andrew Publishing/Noyes; Li and
Kohl (2004) "Complex chemistry & the electroless copper plating
process" Plating & Surface Finishing February 2-8; Xu et al.
(2004) "Preparation of copper nanoparticles on carbon nanotubes by
electroless plating method" Materials Research Bulletin
39:1499-1505; and Siperko (1991) "Scanning tunneling microscopy
studies of Pd--Sn catalyzed electroless Cu deposited on pyrolytic
graphite" J Vac Sci Technol A 9(3):1457-1460. Achieving consistent
and reproducible deposition of discrete nanoparticles (rather than
a continuous or semi-continuous film) on the surface of the
substrate is even more challenging.
[0128] In one aspect, the present invention overcomes the above
noted difficulties by providing methods of producing copper-based
nanoparticles on any of a variety of substrates. The amount of the
copper source present in the plating solution is much lower than is
typical in conventional plating processes. In addition, in contrast
to conventional techniques, the copper source is completely
depleted from the bath after deposition. Deposition of copper at a
high copper concentration requires careful monitoring of time and
reactant concentrations and replenishment of the bath components;
deposition at low concentration, as described herein, provides more
convenient control over the reaction.
[0129] The inventive methods have a number of advantages. For
example, since a known amount of the copper source is added to the
plating bath and substantially completely depleted from the bath by
deposition on the substrate, the reaction stops automatically, and
the amount of copper deposited on the substrate is known and
precisely controlled. There is no need to critically control
plating parameters such as time and bath temperature to achieve the
desired percentage of copper deposited. There is no need to analyze
and/or replenish depleted chemicals during the plating process,
which greatly simplifies deposition on particulate substrates,
whose high surface area complicates deposition with conventional
techniques. Where powdered substrates are employed, the substrate
and the plating chemicals can be mixed in various orders to control
the deposition rate and uniformity. Since copper is substantially
completely depleted after deposition, treatment of spent plating
solutions prior to waste disposal is simplified.
[0130] Accordingly, one general class of embodiments provides
methods for producing nanoparticles. In the methods, a substrate is
provided. Also provided is an electroless plating solution that
comprises at most 10 millimolar copper ions (e.g., Cu.sup.2+ and/or
Cu.sup.+). The substrate is immersed in the plating solution, which
is optionally heated to about 60-70.degree. C., whereby the copper
ions from the plating solution form discrete nanoparticles
comprising copper and/or a copper compound on the substrate, until
the plating solution is substantially completely depleted of copper
ions. "Substantially completely depleted of copper ions" means that
less than 5 ppm or even less than 1 ppm copper ion remains in the
solution. No monitoring or analysis of the components of the
plating solution is necessary during the immersion step, and no
reagents need to be added to the plating solution during the
immersion step.
[0131] Suitable substrates include a planar substrate, silicon
wafer, or foil (e.g., a metal foil, e.g., stainless steel foil).
Suitable substrates include nonporous substrates as well as porous
substrates such as those described above, e.g., a mesh, fabric,
e.g., a woven fabric (e.g., a carbon fabric), fibrous mat,
population of particles, sheets, fibers (including, e.g.,
nanofibers), and/or the like. Thus, exemplary substrates include a
plurality of silica particles (e.g., a silica powder), a plurality
of carbon sheets, carbon powder or a plurality of carbon particles,
natural and/or artificial (synthetic) graphite, natural and/or
artificial (synthetic) graphite particles, graphene, graphene
powder or a plurality of graphene particles, carbon fibers, carbon
nanostructures, carbon nanotubes, and carbon black. The substrate
is optionally a carbon-based substrate, for example, a population
of graphite particles.
[0132] In embodiments in which the substrate comprises a population
of particles (e.g., graphite particles), the particles can be of
essentially any desired shape, for example, spherical or
substantially spherical, elongated, oval/oblong, plate-like (e.g.,
plates, flakes, or sheets), and/or the like. Similarly, the
substrate particles can be of essentially any size. Optionally, the
substrate particles have an average diameter between about 0.5
.mu.m and about 50 .mu.m, e.g., between about 0.5 .mu.m and about 2
.mu.m, between about 2 .mu.m and about 10 .mu.m, between about 2
.mu.m and about 5 .mu.m, between about 5 .mu.m and about 50 .mu.m,
between about 10 .mu.m and about 30 .mu.m, between about 10 .mu.m
and about 20 .mu.m, between about 15 .mu.m and about 25 .mu.m,
between about 15 .mu.m and about 20 .mu.m, or about 20 .mu.m. In
one exemplary class of embodiments, the substrate is a population
of graphite particles, and the graphite particle size is about
10-20 .mu.m. In another exemplary class of embodiments, the
substrate is a population of graphite particles, the graphite
particle size is a few .mu.m (e.g., about 2 .mu.m or less), and the
graphite particles are optionally spherical.
[0133] The substrate is typically activated prior to its immersion
in the plating solution. The substrate is optionally activated as
known in the art by soaking it in a solution of a metal salt, e.g.,
by soaking in a solution of PdCl.sub.2 or AgNO.sub.3. Graphite
substrates, however, particularly graphite particles which have a
very high surface area compared to a conventional planar substrate,
are conveniently activated simply by heating them prior to
immersion in the plating solution. Thus, the methods optionally
include activating the substrate (e.g., a population of graphite
particles) by heating it to 20.degree. C. or more prior to
immersing it in the plating solution. Optionally, the substrate is
activated by heating it to 30.degree. C. or more, preferably
40.degree. C. or more, 50.degree. C. or more, or 60.degree. C. or
more prior to immersion.
[0134] In embodiments in which the substrate comprises a population
of particles, the methods can include filtering the plating
solution to recover the substrate particles from the plating
solution after the plating solution is substantially completely
depleted of copper ions. In general, the substrate is typically
removed from the plating solution after deposition is complete
(that is, after the plating solution is substantially completely
depleted of copper ions).
[0135] As described above, electroless plating generally involves
chemical reduction from an aqueous plating solution that includes
one or more copper source, complexing or chelating agent, and
reducing agent, optionally at alkaline pH. The copper source is
typically a copper salt, e.g., a copper (II) salt (for example,
copper sulfate, copper nitrate, copper (II) chloride (CuCl.sub.2),
or copper acetate) or a copper (I) salt (for example, copper (I)
chloride (CuCl)). Exemplary chelating agents include, but are not
limited to, Rochelle salt, EDTA, and polyols (e.g., Quadrol.RTM.
(N,N,N',N'-tetrakis (2-hydroxypropyl) ethylene-diamine)). Exemplary
reducing agents include, but are not limited to, formaldehyde and
sodium hypophosphite (NaH.sub.2PO.sub.2). The plating solution
optionally includes one or more additives such as a stabilizer,
surfactant, and/or accelerator. The pH of the plating solution can
be adjusted as is well known in the art, for example, by addition
of sodium hydroxide (NaOH), typically to a pH of 12 to 13. The
plating solution is optionally heated, e.g., to a temperature of
60-70.degree. C.
[0136] In one exemplary class of embodiments, the plating solution
comprises a copper (II) salt (e.g., less than 1 g/L anhydrous
copper sulfate), Rochelle salt, and formaldehyde and has an
alkaline pH. In this class of embodiments, the substrate optionally
comprises a population of particles (e.g., graphite particles).
[0137] As noted, when the substrate is initially immersed in the
plating solution, the plating solution comprises at most 10
millimolar copper ions. Optionally, the plating solution comprises
at most 8 millimolar copper ions, at most 6 millimolar copper ions,
at most 4 millimolar copper ions, or at most 2 millimolar copper
ions. It will be evident that the plating solution initially
comprises substantially more than 5 ppm copper ions; for example,
the initial concentration of copper ions can be at least 2
millimolar, e.g., at least 4 millimolar, at least 6 millimolar, or
at least 8 millimolar.
[0138] As noted, the resulting nanoparticles can include copper or
a copper compound (for example, copper oxide). In one class of
embodiments, the nanoparticles comprise elemental copper (Cu),
copper (I) oxide (Cu.sub.2O), copper (II) oxide (CuO), or a
combination thereof. Since copper oxidizes readily into copper
oxide in air, where copper nanoparticles are deposited the
nanoparticles can also contain at least some copper oxide unless
protected from oxidation after the deposition.
[0139] The resulting nanoparticles optionally have an average
diameter between about 5 nm and about 100 nm, e.g., between about
10 nm and about 100 nm, between about 20 nm and about 50 nm, or
between about 20 nm and about 40 nm. Optionally, the nanoparticles
have an average diameter of about 20 nm. The nanoparticles can be
of essentially any shape, but are typically irregularly shaped.
[0140] The resulting nanoparticles are optionally employed as
catalyst particles for subsequent synthesis of other
nanostructures, e.g., nanowires. Thus, the methods can include,
after the plating solution is substantially completely depleted of
copper ions, removing the substrate from the plating solution and
then growing nanostructures (e.g., nanowires, e.g., silicon
nanowires) from the nanoparticles on the substrate. See, for
example, FIG. 3 Panels A and B, which show copper nanoparticles
deposited on a graphite particle substrate from an electroless
plating solution and silicon nanowires grown from the copper
nanoparticles.
[0141] Essentially all of the features noted for the embodiments
above apply to these embodiments as well, as relevant; for example,
with respect to nanostructure growth technique (e.g., VLS or VSS),
type, composition, and size of the resulting nanostructures, ratio
of nanostructures to substrate (e.g., silicon to graphite) by
weight, incorporation into a battery slurry, battery anode, or
battery, and/or the like.
[0142] The plating solution can be employed as a single use bath or
as a reusable bath. Thus, in one class of embodiments, after the
solution is prepared, the substrate is immersed in the solution
until the plating solution is substantially completely depleted of
copper ions, and the solution is then disposed of (after any
necessary treatment to render it safe for disposal) rather than
being replenished and reused for a second substrate. In other
embodiments, however, the bath is reused.
[0143] Thus, in one class of embodiments, after the plating
solution is substantially completely depleted of copper ions, the
substrate is removed from the plating solution, then copper ions
are added to the plating solution, and then a second substrate is
immersed in the plating solution. Typically, the same type and
amount of copper source are added to replenish the reagents that
were used to prepare the solution initially. Thus, as one example,
adding copper ions to the plating solution can comprise adding a
copper (II) salt to the plating solution; additional exemplary
copper sources are listed hereinabove. Typically, after addition of
the copper ions, the plating solution again comprises at most 10
millimolar copper ions. The second substrate is typically but need
not be of the same type as the first substrate, e.g., a second
population of particles, e.g., graphite particles.
[0144] Where the plating solution is to be reused a large number of
times, the concentration of reducing agent and/or chelating agent
and the pH can be analyzed and adjusted if necessary. Where only a
short bath life is needed, however, or where the plating solution
is used only once, there is no need to analyze the solution and
replenish depleted reagents (other than the copper source, if the
solution is used more than once). The shorter bath life results in
ease of operation.
[0145] Since the copper source is nearly completely depleted after
deposition, disposal of used plating solution is simplified
compared to conventional techniques in which a large amount of
copper remains in the used solution. Waste neutralization can be
performed in situ. For example, in embodiments in which the plating
solution comprises formaldehyde, after the plating solution is
substantially completely depleted of copper ions the formaldehyde
can be treated by addition of sodium sulfite to the plating
solution prior to disposing of the solution. The treatment with
sodium sulfite can be performed before or after removal of the
substrate from the plating solution, e.g., before or after
filtration to recover a particulate substrate. The waste can then
be safely disposed of after pH neutralization.
[0146] Additional information on treatment of plating solutions
prior to disposal is available in the art. See, e.g., Capaccio
(1990) "Wastewater treatment for electroless plating" in Mallory
and Hajdu (Eds.) Electroless Plating--Fundamentals and Applications
(pp. 519-528) William Andrew Publishing/Noyes.
[0147] As noted above, nanoparticles produced by electroless
deposition can be employed as catalyst particles in subsequent
nanostructure synthesis reactions. Accordingly, one general class
of embodiments provides methods for producing nanostructures (e.g.,
nanowires). In the methods, a substrate is provided. An electroless
plating solution comprising copper ions is also provided, and the
substrate is immersed in the plating solution, which is optionally
heated to about 60-70.degree. C., whereby the copper ions from the
plating solution form discrete nanoparticles comprising copper
and/or a copper compound on the substrate. Nanostructures (e.g.,
nanowires, e.g., silicon nanowires) are then grown from the
nanoparticles on the substrate. Typically the substrate is removed
from the plating solution prior to growth of the
nanostructures.
[0148] As detailed above, limiting the concentration of copper ions
in the plating solution can be advantageous, particularly for
deposition on porous and/or particulate substrates having high
surface areas. Thus, optionally the plating solution comprises at
most 10 millimolar copper ions (e.g., Cu.sup.2+ and/or Cu.sup.+).
Essentially all of the features noted for the embodiments above
apply to these embodiments as well, as relevant; for example, with
respect to type and composition of substrate (nonporous, porous,
particles, graphite particles, sheets, wafers, etc.), activation of
the substrate, size, shape, and composition of the nanoparticles
(e.g., elemental copper and/or copper oxide), components of the
plating solution (copper source and reducing, chelating, and other
reagents), filtration step to recover a particulate substrate,
reuse versus single use of the plating solution, nanostructure
growth technique (e.g., VLS or VSS), type, composition, and size of
the resulting nanostructures, ratio of nanostructures to substrate
(e.g., silicon to graphite) by weight, incorporation into a battery
slurry, battery anode, or battery, and/or the like.
[0149] Analogous methods to those detailed for copper apply to
electroless deposition of nickel nanoparticles. Such methods are
also a feature of the invention. Accordingly, one general class of
embodiments provides methods for producing nanoparticles. In the
methods, a substrate is provided. Also provided is an electroless
plating solution that comprises at most 10 millimolar nickel ions.
The substrate is immersed in the plating solution, whereby the
nickel ions from the plating solution form discrete nanoparticles
comprising nickel and/or a nickel compound on the substrate, until
the plating solution is substantially completely depleted of nickel
ions. Essentially all of the features noted for the embodiments
above apply to these embodiments as well, as relevant.
Formation of Copper-Based Nanoparticles Through Adsorption
[0150] Nanoparticles can be conveniently formed by adsorption of
copper ions or a copper complex onto the surface of a substrate.
Adsorption is generally defined as the process through which a
substance initially present in one phase (e.g., a liquid) is
removed from that phase by accumulation at the interface between
that phase and a separate phase (e.g., a solid). Adsorption is a
physical separation process in which the adsorbed material is not
chemically altered. Nanoparticles produced via adsorption can be
employed as catalysts for subsequent nanostructure growth.
[0151] Accordingly, one general class of embodiments provides
methods for producing nanostructures (e.g., nanowires). In the
methods, a substrate is provided. A solution comprising copper ions
and/or a copper complex is also provided, and the substrate is
immersed in the solution, whereby the copper ions and/or the copper
complex are adsorbed on the surface of the substrate, thereby
forming discrete nanoparticles comprising a copper compound on the
surface of the substrate. Nanostructures (e.g., nanowires, e.g.,
silicon nanowires) are then grown from the nanoparticles on the
substrate. Typically the substrate is removed from the solution
prior to growth of the nanostructures.
[0152] The solution optionally includes a copper (I) salt or a
copper (II) salt, for example, copper sulfate, copper acetate, or
copper nitrate. The solution can include a copper complex
comprising a chelating agent (a polydentate ligand that forms two
or more coordinate bonds to the metal in the complex), for example,
copper (II) tartrate or copper ethylenediaminetetraacetate (EDTA).
The copper complex can be preformed prior to its addition to the
solution, or it can be formed in the solution, for example, by
mixing a copper salt and a chelating agent (e.g., Rochelle salt and
a copper (II) salt to form copper (II) tartrate). Employing a
copper complex, particularly a complex comprising an organic or
other nonpolar ligand that has stronger van der Waals interactions
with the surface of the substrate than do copper ions, typically
results in greater adsorption on carbon-based substrates than is
seen with uncomplexed copper ions.
[0153] In one class of embodiments the solution is an aqueous
solution, typically, an alkaline solution. Optionally, the solution
has a pH of 12 to 13. In another class of embodiments, the solution
includes an organic solvent (e.g., hexane) instead of water.
[0154] As noted above, the copper ion or complex does not undergo
any chemical reaction when it is adsorbed to form the
nanoparticles. Thus the solution does not include a reducing agent,
in contrast to the plating solution employed in the electroless
deposition techniques described above.
[0155] Suitable substrates include a planar substrate, nonporous
substrate, silicon wafer, or foil (e.g., a metal foil, e.g.,
stainless steel foil), but more preferably include porous
substrates having high surface areas such as those described above,
e.g., a mesh, fabric, e.g., a woven fabric (e.g., a carbon fabric),
fibrous mat, population of particles, sheets, fibers (including,
e.g., nanofibers), and/or the like. Thus, exemplary substrates
include a plurality of silica particles (e.g., a silica powder), a
plurality of carbon sheets, carbon powder or a plurality of carbon
particles, natural and/or artificial (synthetic) graphite, natural
and/or artificial (synthetic) graphite particles, graphene,
graphene powder or a plurality of graphene particles, carbon
fibers, carbon nanostructures, carbon nanotubes, and carbon black.
The substrate is optionally a carbon-based substrate, for example,
a population of graphite particles.
[0156] In embodiments in which the substrate comprises a population
of particles (e.g., graphite particles), the particles can be of
essentially any desired shape, for example, spherical or
substantially spherical, elongated, oval/oblong, plate-like (e.g.,
plates, flakes, or sheets), and/or the like. Similarly, the
substrate particles can be of essentially any size. Optionally, the
substrate particles have an average diameter between about 0.5
.mu.m and about 50 .mu.m, e.g., between about 0.5 .mu.m and about 2
.mu.m, between about 2 .mu.m and about 10 .mu.m, between about 2
.mu.m and about 5 .mu.m, between about 5 .mu.m and about 50 .mu.m,
between about 10 .mu.m and about 30 .mu.m, between about 10 .mu.m
and about 20 .mu.m, between about 15 .mu.m and about 25 mm, between
about 15 .mu.m and about 20 mm, or about 20 mm. In one exemplary
class of embodiments, the substrate is a population of graphite
particles, and the graphite particle size is about 10-20 mm. In
another exemplary class of embodiments, the substrate is a
population of graphite particles, the graphite particle size is a
few .mu.m (e.g., about 2 .mu.m or less), and the graphite particles
are optionally spherical.
[0157] In embodiments in which the substrate comprises a population
of particles, the methods can include filtering the solution to
recover the substrate particles from the solution after deposition
is complete (e.g., after the nanoparticles have reached a desired
size, the desired amount of copper has been adsorbed on the
substrate, or the solution is substantially completely depleted of
copper ions and/or complex).
[0158] The solution is optionally heated to a temperature at which
the ligand in the copper complex is stable, e.g., to 60-70.degree.
C., to increase adsorption. After formation of the nanoparticles
and removal of the substrate from the solution, the substrate can
be further heated, e.g., to a temperature above which the ligand in
the complex is stable, to decompose the copper compound
constituting the nanoparticles and yield copper oxide or (if
heating is performed in a reducing atmosphere) elemental copper
nanoparticles. Such heating can be a separate step, but more
typically occurs in the course of nanostructure synthesis from the
nanoparticles.
[0159] The concentration of copper ions and/or complex in the
solution can be varied as desired. However, limiting the amount of
copper present in the solution can be advantageous, since disposal
of used solution is simplified if copper is substantially
completely depleted from the solution by formation of the
nanoparticles, as noted for the electroless deposition techniques
above. Thus, in one class of embodiments, the solution comprises at
most 10 millimolar copper ions or atoms. Optionally, the solution
comprises at most 8 millimolar copper, at most 6 millimolar copper,
at most 4 millimolar copper, or at most 2 millimolar copper. It
will be evident that the solution initially comprises substantially
more than 5 ppm copper; for example, the initial concentration of
copper ions or atoms can be at least 2 millimolar, e.g., at least 4
millimolar, at least 6 millimolar, or at least 8 millimolar.
[0160] The resulting nanoparticles optionally have an average
diameter between about 1 nm and about 100 nm, e.g., between about 5
nm and about 100 nm, between about 10 nm and about 100 nm, between
about 20 nm and about 50 nm, or between about 20 nm and about 40
nm. Optionally, the nanoparticles have an average diameter of about
20 nm. The nanoparticles can be of essentially any shape, but are
typically irregularly shaped.
[0161] Essentially all of the features noted for the embodiments
above apply to these embodiments as well, as relevant; for example,
with respect to nanostructure growth technique (e.g., VLS or VSS),
type, composition, and size of the resulting nanostructures, ratio
of nanostructures to substrate (e.g., silicon to graphite) by
weight, incorporation into a battery slurry, battery anode, or
battery, and/or the like.
[0162] FIG. 5 Panel A shows copper nanoparticles deposited on
graphite particles by electroless deposition (row I) and adsorption
(row II). FIG. 5 Panel B shows silicon nanowires grown from the
nanoparticles; nanowire morphology and coverage on the graphite is
similar for the different deposition methods.
Nanostructure Growth Using Core-Shell Catalyst Materials
[0163] As noted above, gold (Au) nanoparticles have been
extensively used for Si nanowire growth through the VLS mechanism.
In this mechanism, as schematically illustrated in FIG. 4 Panel A,
the Si from silanes in the vapor phase dissolves into mediating Au
nanoparticles 401 deposited on substrate 400, to form Au--Si
eutectic catalyst droplets 402. As the amount of Si in the Au--Si
alloy droplet increases, the concentration of Si eventually goes
beyond saturation. The Si then evolves from liquid droplet 403 to
solid 404 and Si nanowires are formed. Solidified Au nanoparticles
at the tips of the resulting nanowires are therefore typical
characteristics of VLS-grown nanowires. The diameter of the Si
nanowires is determined by the diameter of the Au
nanoparticles.
[0164] With increasing demand for silicon nanowires, the cost of
gold nanoparticles becomes more significant, potentially limiting
the use of silicon nanowires for applications such as batteries and
medical devices. The cost of the gold nanoparticles is mostly based
on the materials price of gold. Methods described above focus on
entirely replacing the gold catalyst with other materials. Another
approach, however, is to decrease the amount of gold required.
[0165] Accordingly, one aspect of the present invention features
methods of producing nanostructures (e.g., nanowires, e.g., silicon
nanowires) in which nanoparticles that have a non-Au core
encapsulated by an Au shell are used in place of solid Au
nanoparticles as the catalyst particles. As schematically
illustrated in FIG. 4 Panel B, catalyst particles 451 deposited on
substrate 450 comprise non-Au core 462 and Au shell 461. During VLS
growth when a non-Au core is present in the system, the
liquid-solid interface differs from that for a solid gold catalyst.
For ease of explanation, it is assumed that the non-Au core does
not react with Au and Si and thus the eutectic nature of Au--Si
does not change. Thus, during the growth process, Si from silanes
in the vapor phase dissolves into the Au from the shell to form
Au--Si eutectic catalyst droplets 452 still containing the non-Au
core 462. As the amount of Si in the Au--Si alloy droplet
increases, the concentration of Si eventually goes beyond
saturation. The Si then evolves from liquid droplet 453 to solid
454 and Si nanowires are formed. Both gold and the non-gold core
material are present at the tip of the resulting nanowire. For core
materials where reaction between the core material, Au, and Si does
occur, phase diagrams of the new ternary gold alloys can be
explored.
[0166] The Au atoms forming the shell react with Si atoms to form
the eutectic alloy. Without limitation to any particular mechanism,
the presence of non-Au alien species at the interface of the
eutectic droplet and Si allows Si nanowires with comparable
diameter to be grown from the core shell catalyst particles as from
single solid Au particles, while requiring less gold per nanowire.
Thus, some percentage of Au can be replaced by the non-Au
component, reducing the cost of nanowire synthesis. (Typically, the
non-Au core material is much less expensive than is Au.)
[0167] As one specific example of how much gold can be saved by
employing non-Au core/Au shell nanoparticles as catalysts for
nanostructure synthesis, FIG. 4 Panel C represents the percentage
of the nanoparticle volume occupied by the non-Au material (i.e.,
the volume of the core as a percentage of the overall volume
including both the core and the shell) for a 15 nm non-Au core
coated with an Au shell of varying thickness. As can be seen from
the graph, up to 82.4% of Au can be saved if 15 nm non-Au core
particles covered with a 1 nm Au shell are used for Si nanowire
growth. This percentage savings decreases as thicker Au shells are
used.
[0168] As noted, the core of the nanoparticles comprises a material
other than gold. Exemplary materials for the core include, but are
not limited to: metal oxides, for example, Fe.sub.2O.sub.3,
MFe.sub.2O.sub.4 (where M is, e.g., Fe, Mn, Mg, Co, or Ni)
particularly spinel type, TiO.sub.2, Al.sub.2O.sub.3, and ZnO;
metals, for example, Fe, Ni, Co, Pd, and Ag; and non-metal oxides,
for example, SiO.sub.2 and other silicates such as polyhedral
oligomeric silsesquioxanes. The material constituting the core can,
but need not, form an alloy with gold. In embodiments in which the
material constituting the core forms an alloy with gold, the alloy
can but need not exhibit a eutectic point. Silver, for example,
forms an alloy with gold but there is no eutectic phase. In the
example described above for silicon nanowire growth, Au is needed
at the surface to form an Au--Si alloy in the VLS growth mechanism.
However, the core material can be designed to form an alloy with Au
at one condition and at another condition not to form the alloy. In
embodiments in which the formed alloy can also work as a catalyst
for growth of other nanostructures such as nanotubes, nanobelts,
and the like, the processing conditions can be controlled to form
chimeric nanostructures between, e.g., silicon nanowires and other
nanostructures.
[0169] The core is optionally between about 5 nm and about 500 nm
in diameter, e.g., between about 10 nm and about 500 nm, between
about 5 nm and about 100 nm, between about 10 nm and about 100 nm,
between about 5 nm and about 40 nm, between about 5 nm and about 20
nm, or between about 5 nm and about 10 nm in diameter. The
thickness of the gold shell is optionally between about 1 nm and
about 50 nm, e.g., between about 1 nm and about 40 nm, between
about 1 nm and about 20 nm, between about 1 nm and about 10 nm, or
between about 1 nm and about 5 nm.
[0170] In one class of embodiments, nanowires are grown from the
non Au core-Au shell catalyst particles. Optionally, the nanowires
are hollow. Suitable materials for the nanostructures (e.g.,
nanowires) include, e.g., silicon and/or germanium, as well as the
inorganic conductive or semiconductive materials noted
hereinabove.
[0171] Essentially all of the features noted for the embodiments
above apply to these embodiments as well, as relevant; for example,
with respect to type, composition, and size of the resulting
nanostructures, type and composition of substrate (nonporous,
porous, particles, graphite particles, sheets, wafers, etc.), ratio
of nanostructures to substrate (e.g., silicon to graphite) by
weight, incorporation into a battery slurry, battery anode, or
battery, and/or the like.
[0172] In a related aspect, nanoparticles that have an Au core
encapsulated by a non-Au shell are used as catalyst particles for
nanostructure synthesis. Exemplary materials for the shell include,
but are not limited to: metal oxides, for example, Fe.sub.2O.sub.3,
MFe.sub.2O.sub.4 (where M is, e.g., Fe, Mn, Mg, Co, or Ni)
particularly spinel type, TiO.sub.2, Al.sub.2O.sub.3, and ZnO;
metals, for example, Fe, Ni, Co, Pd, and Ag; and non-metal oxides,
for example, SiO.sub.2 and other silicates such as polyhedral
oligomeric silsesquioxanes. The material constituting the shell
can, but need not, form an alloy with gold. In embodiments in which
the material constituting the shell forms an alloy with gold, the
alloy can but need not exhibit a eutectic point.
[0173] Use of Au core-non Au shell catalyst particles, like use of
non Au core-Au shell catalyst particles described above, can be
advantageous in reducing the amount of gold required for
nanostructure synthesis. It can also offer additional advantages,
for example, by facilitating synthesis of core-shell
nanostructures. For example, nanostructures having an Au core and a
Ni shell can be employed as catalyst particles. Core and shell
sizes of the catalyst particles are selected to produce an
appropriate composition for the alloy (e.g., 20% Au and 80% Ni),
and nanostructure synthesis is performed under appropriate
conditions with different precursors (e.g., 20% silane and 80%
ethylene) to produce nanostructures (e.g., nanowires) having a
silicon core and a carbon shell.
[0174] Essentially all of the features noted for the embodiments
above apply to these embodiments as well, as relevant; for example,
with respect to size of the core, thickness of the shell, type,
composition, and size of the resulting nanostructures, type and
composition of substrate (nonporous, porous, particles, graphite
particles, sheets, wafers, etc.), ratio of nanostructures to
substrate (e.g., silicon to graphite) by weight, incorporation into
a battery slurry, battery anode, or battery, and/or the like.
EXAMPLES
[0175] It is understood that the examples and embodiments described
herein are for illustrative purposes only and that various
modifications or changes in light thereof will be suggested to
persons skilled in the art and are to be included within the spirit
and purview of this application and scope of the appended claims.
Accordingly, the following examples are offered to illustrate, but
not to limit, the claimed invention.
Example 1
Synthesis of Colloidal Cu.sub.2O, Deposition of Cu.sub.2O
Nanoparticles on Graphite Particles, and Growth of Silicon
Nanowires From the Cu.sub.2O Catalyst
[0176] Cu.sub.2O colloidal nanoparticles are prepared as follows.
Solution A is prepared by mixing 18 ml deionized water, 2 ml 0.1M
copper sulfate, and 30 ml 0.01M CTAB (cetrimonium bromide).
Solution B is prepared by mixing 48 ml deionized water, 1 ml 0.5M
sodium ascorbate, and 1 ml 5M sodium hydroxide. Solution B is added
to solution A while stirring, and the mixture turns to golden
yellow almost instantaneously. The Cu.sub.2O colloid synthesized is
stable for at least a few hours and has an average diameter of
about 45 nm based on light scattering measurements.
[0177] The Cu.sub.2O colloidal nanoparticles are then deposited on
graphite substrate particles as follows. In another beaker, about
10 g synthetic graphite powder having an average diameter of about
10 .mu.m is mixed with 100 ml deionized water and agitated for at
least 5 minutes using a magnetic stirring bar at a speed of 400
rpm. The graphite slurry is then heated to about 65.degree. C.
under constant stirring. At about 65.degree. C., all the Cu.sub.2O
colloid synthesized above is slowly added to the graphite slurry,
and the solution temperature is maintained at 60-65.degree. C. for
30 minutes. Then the slurry is filtered under vacuum through a
filtration apparatus having a pore size of 0.2 .mu.m and rinsed
with copious amount of deionized water. The graphite cake is
removed from the filter and then dried in an oven at 120.degree. C.
for at least 12 hours. The effluent collected in the filtration
apparatus is analyzed for copper concentration, which is about 10
ppm. Cu.sub.2O nanoparticles are adsorbed on the graphite particles
and the amount of copper is estimated to be 0.1% by weight (based
on the amount of copper remaining in the effluent). See, e.g., FIG.
2 Panel A, which shows nanoparticles deposited on graphite
particles under conditions similar to those described in this
example.
[0178] Silicon nanowires are then synthesized from the Cu.sub.2O
catalyst particles on the graphite substrate as follows. 10 g
graphite powder with the Cu.sub.2O catalyst is loaded in a quartz
cup with its bottom comprising a thin sheet of carbon paper and a
disc of quartz frit, both of which are permeable to gas but not to
the graphite particles. The quartz cup is covered with a quartz lid
which is connected to the gas inlet port of a hot-wall CVD reactor.
The reactor is ramped to a temperature of 460.degree. C., initially
under vacuum and later in hydrogen flowing at 200 sccm or higher.
Silicon nanowires are grown at 45 Torr having a partial pressure of
1-4 Torr in silane, which is diluted by hydrogen and/or helium.
[0179] The graphite particles, after 45 minutes of growth, gain
about 5-10% weight. The top layer of the graphite bed is lighter in
color than the bottom layer due to gas depletion effect, which may
be mitigated by flowing the reactant gases in both upward and
downward directions or employing a reaction system in which the
graphite particles and the reactant gases are mixed more evenly
throughout the growth process. Silicon nanowires grown on the
graphite particles have an average diameter between 10 nm and 100
nm and are essentially crystalline due to the nature of the VSS
growth mechanism. Some nanowires are straight but most are kinked
(see, e.g., FIG. 2 Panel B, which shows nanowires grown under
conditions similar to those described in this example). One end of
the nanowire is attached to the graphite surface and the other end
has a Cu.sub.3Si (copper silicide) tip.
Example 2
Electroless Deposition of Copper on Graphite Particles and Growth
of Silicon Nanowires From the Copper Catalyst
[0180] About 10 g synthetic graphite powder having an average
diameter of about 10 .mu.m is mixed with 100 ml deionized water in
a glass beaker and agitated for at least 5 minutes using a magnetic
stirring bar at a speed of 400 rpm. 1 ml stock solution having 0.2M
copper sulfate and 5.3M formaldehyde is pipetted into the graphite
slurry, and heated to about 65.degree. C. under constant stirring.
At 60.degree. C. or higher, 1 ml stock solution having 0.4M
Rochelle salt and 5M sodium hydroxide is added by pipet. After a 30
minute reaction at 60-65.degree. C., the slurry is filtered under
vacuum through a filtration apparatus having a pore size of 0.2
.mu.m and rinsed with copious amount of deionized water.
Optionally, the residual formaldehyde in the plating solution is
treated with 0.6 g sodium sulfite either before or after the
filtration step. The graphite cake is removed from the filter and
then dried in an oven at 120.degree. C. for at least 12 hours. The
effluent collected in the filtration apparatus is analyzed for
copper concentration, which is typically 1 ppm or below. The amount
of copper catalyst in the graphite powder is estimated to be 0.12%
by weight (based on the amount of copper remaining in the
effluent). Copper nanoparticles vary in size between 10 nm to 100
nm and are distributed fairly evenly on the graphite particles.
See, e.g., FIG. 3 Panel A, which shows nanoparticles produced under
conditions similar to those described in this example.
[0181] Silicon nanowires are grown from the copper nanoparticles on
the graphite particles under the same conditions as in Example 1,
with similar results in weight gain and nanowire morphology. See,
e.g., FIG. 3 Panel B, which shows nanowires grown under conditions
similar to those described in this example.
Example 3
Adsorption of Copper Complexes on Graphite Particles and Growth of
Silicon Nanowires From the Copper Catalyst
[0182] About 10 g synthetic graphite powder having an average
diameter of about 10 .mu.m is mixed with 100 ml deionized water in
a glass beaker and agitated for at least 5 minutes using a magnetic
stirring bar at a speed of 400 rpm. 1 ml 0.2M copper sulfate
solution is added by pipet to the graphite slurry and heated to
about 65.degree. C. under constant stirring. At 60.degree. C. or
higher, 1 ml stock solution having 0.4M Rochelle salt and 5M sodium
hydroxide is added by pipet. After a 30 minute reaction at
60-65.degree. C., the slurry is filtered under vacuum through a
filtration apparatus having a pore size of 0.2 .mu.m and rinsed
with copious amount of deionized water. The graphite cake is
removed from the filter and then dried in an oven at 120.degree. C.
for at least 12 hours. The effluent collected in the filtration
apparatus is analyzed for copper concentration, which is typically
1-2 ppm or below. The amount of copper catalyst in the graphite
powder is estimated to be 0.12% by weight (based on the amount of
copper remaining in the effluent). Copper nanoparticles vary in
size between 10 nm to 50 nm and are distributed fairly evenly on
the graphite particles. See, e.g., FIG. 5 Panel A, row II, which
shows nanoparticles produced under conditions similar to those
described in this example.
[0183] Silicon nanowires are grown from the copper nanoparticles on
the graphite particles under the same conditions as in Example 1,
with similar results in weight gain and nanowire morphology. See,
e.g., FIG. 5 Panel B, row II, which shows nanowires grown under
conditions similar to those described in this example.
[0184] While the foregoing invention has been described in some
detail for purposes of clarity and understanding, it will be clear
to one skilled in the art from a reading of this disclosure that
various changes in form and detail can be made without departing
from the true scope of the invention. For example, all the
techniques and apparatus described above can be used in various
combinations. All publications, patents, patent applications,
and/or other documents cited in this application are incorporated
by reference in their entirety for all purposes to the same extent
as if each individual publication, patent, patent application,
and/or other document were individually indicated to be
incorporated by reference for all purposes.
* * * * *